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PERLGLOSSARY(1) Perl Programmers Reference Guide PERLGLOSSARY(1)

NAME
perlglossary - Perl Glossary

VERSION
version 5.20210411

DESCRIPTION
A glossary of terms (technical and otherwise) used in the Perl documentation, derived from
the Glossary of Programming Perl, Fourth Edition. Words or phrases in bold are defined
elsewhere in this glossary.

Other useful sources include the Unicode Glossary <http://unicode.org/glossary/>, the Free
On-Line Dictionary of Computing <http://foldoc.org/>, the Jargon File
<http://catb.org/~esr/jargon/>, and Wikipedia <http://www.wikipedia.org/>.

A
accessor methods
A method used to indirectly inspect or update an object’s state (its instance variables).

actual arguments
The scalar values that you supply to a function or subroutine when you call it. For
instance, when you call "power("puff")", the string "puff" is the actual argument. See
also argument and formal arguments.

address operator
Some languages work directly with the memory addresses of values, but this can be like
playing with fire. Perl provides a set of asbestos gloves for handling all memory
management. The closest to an address operator in Perl is the backslash operator, but it
gives you a hard reference, which is much safer than a memory address.

algorithm
A well-defined sequence of steps, explained clearly enough that even a computer could do
them.

alias
A nickname for something, which behaves in all ways as though you’d used the original
name instead of the nickname. Temporary aliases are implicitly created in the loop
variable for "foreach" loops, in the $_ variable for "map" or "grep" operators, in $a and
$b during "sort"’s comparison function, and in each element of @_ for the actual
arguments of a subroutine call. Permanent aliases are explicitly created in packages by
importing symbols or by assignment to typeglobs. Lexically scoped aliases for package
variables are explicitly created by the "our" declaration.

alphabetic
The sort of characters we put into words. In Unicode, this is all letters including all
ideographs and certain diacritics, letter numbers like Roman numerals, and various
combining marks.

alternatives
A list of possible choices from which you may select only one, as in, “Would you like
door A, B, or C?” Alternatives in regular expressions are separated with a single
vertical bar: "|". Alternatives in normal Perl expressions are separated with a double
vertical bar: "||". Logical alternatives in Boolean expressions are separated with either
"||" or "or".

anonymous
Used to describe a referent that is not directly accessible through a named variable.
Such a referent must be indirectly accessible through at least one hard reference. When
the last hard reference goes away, the anonymous referent is destroyed without pity.

application
A bigger, fancier sort of program with a fancier name so people don’t realize they are
using a program.

architecture
The kind of computer you’re working on, where one “kind of computer” means all those
computers sharing a compatible machine language. Since Perl programs are (typically)
simple text files, not executable images, a Perl program is much less sensitive to the
architecture it’s running on than programs in other languages, such as C, that are
compiled into machine code. See also platform and operating system.

argument
A piece of data supplied to a program, subroutine, function, or method to tell it what
it’s supposed to do. Also called a “parameter”.

ARGV
The name of the array containing the argument vector from the command line. If you use
the empty "<>" operator, "ARGV" is the name of both the filehandle used to traverse the
arguments and the scalar containing the name of the current input file.

arithmetical operator
A symbol such as "+" or "/" that tells Perl to do the arithmetic you were supposed to
learn in grade school.

array
An ordered sequence of values, stored such that you can easily access any of the values
using an integer subscript that specifies the value’s offset in the sequence.

array context
An archaic expression for what is more correctly referred to as list context.

Artistic License
The open source license that Larry Wall created for Perl, maximizing Perl’s usefulness,
availability, and modifiability. The current version is 2.
(<http://www.opensource.org/licenses/artistic-license.php>).

ASCII
The American Standard Code for Information Interchange (a 7-bit character set adequate
only for poorly representing English text). Often used loosely to describe the lowest 128
values of the various ISO-8859-X character sets, a bunch of mutually incompatible 8-bit
codes best described as half ASCII. See also Unicode.

assertion
A component of a regular expression that must be true for the pattern to match but does
not necessarily match any characters itself. Often used specifically to mean a zero-width
assertion.

assignment
An operator whose assigned mission in life is to change the value of a variable.

assignment operator
Either a regular assignment or a compound operator composed of an ordinary assignment and
some other operator, that changes the value of a variable in place; that is, relative to
its old value. For example, "$a += 2" adds 2 to $a.

associative array
See hash. Please. The term associative array is the old Perl 4 term for a hash. Some
languages call it a dictionary.

associativity
Determines whether you do the left operator first or the right operator first when you
have “A operator B operator C”, and the two operators are of the same precedence.
Operators like "+" are left associative, while operators like "**" are right associative.
See Camel chapter 3, “Unary and Binary Operators” for a list of operators and their
associativity.

asynchronous
Said of events or activities whose relative temporal ordering is indeterminate because
too many things are going on at once. Hence, an asynchronous event is one you didn’t know
when to expect.

atom
A regular expression component potentially matching a substring containing one or more
characters and treated as an indivisible syntactic unit by any following quantifier.
(Contrast with an assertion that matches something of zero width and may not be
quantified.)

atomic operation
When Democritus gave the word “atom” to the indivisible bits of matter, he meant
literally something that could not be cut: ἀ- (not) + -τομος (cuttable). An atomic
operation is an action that can’t be interrupted, not one forbidden in a nuclear-free
zone.

attribute
A new feature that allows the declaration of variables and subroutines with modifiers, as
in "sub foo : locked method". Also another name for an instance variable of an object.

autogeneration
A feature of operator overloading of objects, whereby the behavior of certain operators
can be reasonably deduced using more fundamental operators. This assumes that the
overloaded operators will often have the same relationships as the regular operators. See
Camel chapter 13, “Overloading”.

autoincrement
To add one to something automatically, hence the name of the "++" operator. To instead
subtract one from something automatically is known as an “autodecrement”.

autoload
To load on demand. (Also called “lazy” loading.) Specifically, to call an "AUTOLOAD"
subroutine on behalf of an undefined subroutine.

autosplit
To split a string automatically, as the –a switch does when running under –p or –n in
order to emulate awk. (See also the "AutoSplit" module, which has nothing to do with the
"–a" switch but a lot to do with autoloading.)

autovivification
A Graeco-Roman word meaning “to bring oneself to life”. In Perl, storage locations
(lvalues) spontaneously generate themselves as needed, including the creation of any hard
reference values to point to the next level of storage. The assignment "$a[5][5][5][5][5]
= "quintet"" potentially creates five scalar storage locations, plus four references (in
the first four scalar locations) pointing to four new anonymous arrays (to hold the last
four scalar locations). But the point of autovivification is that you don’t have to worry
about it.

AV Short for “array value”, which refers to one of Perl’s internal data types that holds an
array. The "AV" type is a subclass of SV.

awk Descriptive editing term—short for “awkward”. Also coincidentally refers to a venerable
text-processing language from which Perl derived some of its high-level ideas.

B
backreference
A substring captured by a subpattern within unadorned parentheses in a regex. Backslashed
decimal numbers ("\1", "\2", etc.) later in the same pattern refer back to the
corresponding subpattern in the current match. Outside the pattern, the numbered
variables ($1, $2, etc.) continue to refer to these same values, as long as the pattern
was the last successful match of the current dynamic scope.

backtracking
The practice of saying, “If I had to do it all over, I’d do it differently,” and then
actually going back and doing it all over differently. Mathematically speaking, it’s
returning from an unsuccessful recursion on a tree of possibilities. Perl backtracks when
it attempts to match patterns with a regular expression, and its earlier attempts don’t
pan out. See the section “The Little Engine That /Couldn(n’t)” in Camel chapter 5,
“Pattern Matching”.

backward compatibility
Means you can still run your old program because we didn’t break any of the features or
bugs it was relying on.

bareword
A word sufficiently ambiguous to be deemed illegal under "use strict 'subs'". In the
absence of that stricture, a bareword is treated as if quotes were around it.

base class
A generic object type; that is, a class from which other, more specific classes are
derived genetically by inheritance. Also called a “superclass” by people who respect
their ancestors.

big-endian
From Swift: someone who eats eggs big end first. Also used of computers that store the
most significant byte of a word at a lower byte address than the least significant byte.
Often considered superior to little-endian machines. See also little-endian.

binary
Having to do with numbers represented in base 2. That means there’s basically two
numbers: 0 and 1. Also used to describe a file of “nontext”, presumably because such a
file makes full use of all the binary bits in its bytes. With the advent of Unicode, this
distinction, already suspect, loses even more of its meaning.

binary operator
An operator that takes two operands.

bind
To assign a specific network address to a socket.

bit An integer in the range from 0 to 1, inclusive. The smallest possible unit of information
storage. An eighth of a byte or of a dollar. (The term “Pieces of Eight” comes from
being able to split the old Spanish dollar into 8 bits, each of which still counted for
money. That’s why a 25- cent piece today is still “two bits”.)

bit shift
The movement of bits left or right in a computer word, which has the effect of
multiplying or dividing by a power of 2.

bit string
A sequence of bits that is actually being thought of as a sequence of bits, for once.

bless
In corporate life, to grant official approval to a thing, as in, “The VP of Engineering
has blessed our WebCruncher project.” Similarly, in Perl, to grant official approval to a
referent so that it can function as an object, such as a WebCruncher object. See the
"bless" function in Camel chapter 27, “Functions”.

block
What a process does when it has to wait for something: “My process blocked waiting for
the disk.” As an unrelated noun, it refers to a large chunk of data, of a size that the
operating system likes to deal with (normally a power of 2 such as 512 or 8192).
Typically refers to a chunk of data that’s coming from or going to a disk file.

BLOCK
A syntactic construct consisting of a sequence of Perl statements that is delimited by
braces. The "if" and "while" statements are defined in terms of "BLOCK"s, for instance.
Sometimes we also say “block” to mean a lexical scope; that is, a sequence of statements
that acts like a "BLOCK", such as within an "eval" or a file, even though the statements
aren’t delimited by braces.

block buffering
A method of making input and output efficient by passing one block at a time. By default,
Perl does block buffering to disk files. See buffer and command buffering.

Boolean
A value that is either true or false.

Boolean context
A special kind of scalar context used in conditionals to decide whether the scalar value
returned by an expression is true or false. Does not evaluate as either a string or a
number. See context.

breakpoint
A spot in your program where you’ve told the debugger to stop execution so you can poke
around and see whether anything is wrong yet.

broadcast
To send a datagram to multiple destinations simultaneously.

BSD A psychoactive drug, popular in the ’80s, probably developed at UC Berkeley or
thereabouts. Similar in many ways to the prescription-only medication called “System V”,
but infinitely more useful. (Or, at least, more fun.) The full chemical name is “Berkeley
Standard Distribution”.

bucket
A location in a hash table containing (potentially) multiple entries whose keys “hash” to
the same hash value according to its hash function. (As internal policy, you don’t have
to worry about it unless you’re into internals, or policy.)

buffer
A temporary holding location for data. Data that are Block buffering means that the data
is passed on to its destination whenever the buffer is full. Line buffering means that
it’s passed on whenever a complete line is received. Command buffering means that it’s
passed every time you do a "print" command (or equivalent). If your output is unbuffered,
the system processes it one byte at a time without the use of a holding area. This can be
rather inefficient.

built-in
A function that is predefined in the language. Even when hidden by overriding, you can
always get at a built- in function by qualifying its name with the "CORE::"
pseudopackage.

bundle
A group of related modules on CPAN. (Also sometimes refers to a group of command-line
switches grouped into one switch cluster.)

byte
A piece of data worth eight bits in most places.

bytecode
A pidgin-like lingo spoken among ’droids when they don’t wish to reveal their orientation
(see endian). Named after some similar languages spoken (for similar reasons) between
compilers and interpreters in the late 20ᵗʰ century. These languages are characterized by
representing everything as a nonarchitecture-dependent sequence of bytes.

C
C A language beloved by many for its inside-out type definitions, inscrutable precedence
rules, and heavy overloading of the function-call mechanism. (Well, actually, people
first switched to C because they found lowercase identifiers easier to read than upper.)
Perl is written in C, so it’s not surprising that Perl borrowed a few ideas from it.

cache
A data repository. Instead of computing expensive answers several times, compute it once
and save the result.

callback
A handler that you register with some other part of your program in the hope that the
other part of your program will trigger your handler when some event of interest
transpires.

call by reference
An argument-passing mechanism in which the formal arguments refer directly to the actual
arguments, and the subroutine can change the actual arguments by changing the formal
arguments. That is, the formal argument is an alias for the actual argument. See also
call by value.

call by value
An argument-passing mechanism in which the formal arguments refer to a copy of the actual
arguments, and the subroutine cannot change the actual arguments by changing the formal
arguments. See also call by reference.

canonical
Reduced to a standard form to facilitate comparison.

capture variables
The variables—such as $1 and $2, and "%+" and "%– "—that hold the text remembered in a
pattern match. See Camel chapter 5, “Pattern Matching”.

capturing
The use of parentheses around a subpattern in a regular expression to store the matched
substring as a backreference. (Captured strings are also returned as a list in list
context.) See Camel chapter 5, “Pattern Matching”.

cargo cult
Copying and pasting code without understanding it, while superstitiously believing in its
value. This term originated from preindustrial cultures dealing with the detritus of
explorers and colonizers of technologically advanced cultures. See The Gods Must Be
Crazy.

case
A property of certain characters. Originally, typesetter stored capital letters in the
upper of two cases and small letters in the lower one. Unicode recognizes three cases:
lowercase (character property "\p{lower}"), titlecase ("\p{title}"), and uppercase
("\p{upper}"). A fourth casemapping called foldcase is not itself a distinct case, but it
is used internally to implement casefolding. Not all letters have case, and some
nonletters have case.

casefolding
Comparing or matching a string case-insensitively. In Perl, it is implemented with the
"/i" pattern modifier, the "fc" function, and the "\F" double-quote translation escape.

casemapping
The process of converting a string to one of the four Unicode casemaps; in Perl, it is
implemented with the "fc", "lc", "ucfirst", and "uc" functions.

character
The smallest individual element of a string. Computers store characters as integers, but
Perl lets you operate on them as text. The integer used to represent a particular
character is called that character’s codepoint.

character class
A square-bracketed list of characters used in a regular expression to indicate that any
character of the set may occur at a given point. Loosely, any predefined set of
characters so used.

character property
A predefined character class matchable by the "\p" or "\P" metasymbol. Unicode defines
hundreds of standard properties for every possible codepoint, and Perl defines a few of
its own, too.

circumfix operator
An operator that surrounds its operand, like the angle operator, or parentheses, or a
hug.

class
A user-defined type, implemented in Perl via a package that provides (either directly or
by inheritance) methods (that is, subroutines) to handle instances of the class (its
objects). See also inheritance.

class method
A method whose invocant is a package name, not an object reference. A method associated
with the class as a whole. Also see instance method.

client
In networking, a process that initiates contact with a server process in order to
exchange data and perhaps receive a service.

closure
An anonymous subroutine that, when a reference to it is generated at runtime, keeps track
of the identities of externally visible lexical variables, even after those lexical
variables have supposedly gone out of scope. They’re called “closures” because this sort
of behavior gives mathematicians a sense of closure.

cluster
A parenthesized subpattern used to group parts of a regular expression into a single
atom.

CODE
The word returned by the "ref" function when you apply it to a reference to a subroutine.
See also CV.

code generator
A system that writes code for you in a low-level language, such as code to implement the
backend of a compiler. See program generator.

codepoint
The integer a computer uses to represent a given character. ASCII codepoints are in the
range 0 to 127; Unicode codepoints are in the range 0 to 0x1F_FFFF; and Perl codepoints
are in the range 0 to 2³²−1 or 0 to 2⁶⁴−1, depending on your native integer size. In Perl
Culture, sometimes called ordinals.

code subpattern
A regular expression subpattern whose real purpose is to execute some Perl code—for
example, the "(?{...})" and "(??{...})" subpatterns.

collating sequence
The order into which characters sort. This is used by string comparison routines to
decide, for example, where in this glossary to put “collating sequence”.

co-maintainer
A person with permissions to index a namespace in PAUSE. Anyone can upload any namespace,
but only primary and co-maintainers get their contributions indexed.

combining character
Any character with the General Category of Combining Mark ("\p{GC=M}"), which may be
spacing or nonspacing. Some are even invisible. A sequence of combining characters
following a grapheme base character together make up a single user-visible character
called a grapheme. Most but not all diacritics are combining characters, and vice versa.

command
In shell programming, the syntactic combination of a program name and its arguments. More
loosely, anything you type to a shell (a command interpreter) that starts it doing
something. Even more loosely, a Perl statement, which might start with a label and
typically ends with a semicolon.

command buffering
A mechanism in Perl that lets you store up the output of each Perl command and then flush
it out as a single request to the operating system. It’s enabled by setting the $|
($AUTOFLUSH) variable to a true value. It’s used when you don’t want data sitting around,
not going where it’s supposed to, which may happen because the default on a file or pipe
is to use block buffering.

command-line arguments
The values you supply along with a program name when you tell a shell to execute a
command. These values are passed to a Perl program through @ARGV.

command name
The name of the program currently executing, as typed on the command line. In C, the
command name is passed to the program as the first command-line argument. In Perl, it
comes in separately as $0.

comment
A remark that doesn’t affect the meaning of the program. In Perl, a comment is
introduced by a "#" character and continues to the end of the line.

compilation unit
The file (or string, in the case of "eval") that is currently being compiled.

compile
The process of turning source code into a machine-usable form. See compile phase.

compile phase
Any time before Perl starts running your main program. See also run phase. Compile phase
is mostly spent in compile time, but may also be spent in runtime when "BEGIN" blocks,
"use" or "no" declarations, or constant subexpressions are being evaluated. The startup
and import code of any "use" declaration is also run during compile phase.

compiler
Strictly speaking, a program that munches up another program and spits out yet another
file containing the program in a “more executable” form, typically containing native
machine instructions. The perl program is not a compiler by this definition, but it does
contain a kind of compiler that takes a program and turns it into a more executable form
(syntax trees) within the perl process itself, which the interpreter then interprets.
There are, however, extension modules to get Perl to act more like a “real” compiler. See
Camel chapter 16, “Compiling”.

compile time
The time when Perl is trying to make sense of your code, as opposed to when it thinks it
knows what your code means and is merely trying to do what it thinks your code says to
do, which is runtime.

composer
A “constructor” for a referent that isn’t really an object, like an anonymous array or a
hash (or a sonata, for that matter). For example, a pair of braces acts as a composer
for a hash, and a pair of brackets acts as a composer for an array. See the section
“Creating References” in Camel chapter 8, “References”.

concatenation
The process of gluing one cat’s nose to another cat’s tail. Also a similar operation on
two strings.

conditional
Something “iffy”. See Boolean context.

connection
In telephony, the temporary electrical circuit between the caller’s and the callee’s
phone. In networking, the same kind of temporary circuit between a client and a server.

construct
As a noun, a piece of syntax made up of smaller pieces. As a transitive verb, to create
an object using a constructor.

constructor
Any class method, instance, or subroutine that composes, initializes, blesses, and
returns an object. Sometimes we use the term loosely to mean a composer.

context
The surroundings or environment. The context given by the surrounding code determines
what kind of data a particular expression is expected to return. The three primary
contexts are list context, scalar, and void context. Scalar context is sometimes
subdivided into Boolean context, numeric context, string context, and void context.
There’s also a “don’t care” context (which is dealt with in Camel chapter 2, “Bits and
Pieces”, if you care).

continuation
The treatment of more than one physical line as a single logical line. Makefile lines are
continued by putting a backslash before the newline. Mail headers, as defined by RFC 822,
are continued by putting a space or tab after the newline. In general, lines in Perl do
not need any form of continuation mark, because whitespace (including newlines) is
gleefully ignored. Usually.

core dump
The corpse of a process, in the form of a file left in the working directory of the
process, usually as a result of certain kinds of fatal errors.

CPAN
The Comprehensive Perl Archive Network. (See the Camel Preface and Camel chapter 19,
“CPAN” for details.)

C preprocessor
The typical C compiler’s first pass, which processes lines beginning with "#" for
conditional compilation and macro definition, and does various manipulations of the
program text based on the current definitions. Also known as cpp(1).

cracker
Someone who breaks security on computer systems. A cracker may be a true hacker or only a
script kiddie.

currently selected output channel
The last filehandle that was designated with "select(FILEHANDLE)"; "STDOUT", if no
filehandle has been selected.

current package
The package in which the current statement is compiled. Scan backward in the text of your
program through the current lexical scope or any enclosing lexical scopes until you find
a package declaration. That’s your current package name.

current working directory
See working directory.

CV In academia, a curriculum vitæ, a fancy kind of résumé. In Perl, an internal “code value”
typedef holding a subroutine. The "CV" type is a subclass of SV.

D
dangling statement
A bare, single statement, without any braces, hanging off an "if" or "while" conditional.
C allows them. Perl doesn’t.

datagram
A packet of data, such as a UDP message, that (from the viewpoint of the programs
involved) can be sent independently over the network. (In fact, all packets are sent
independently at the IP level, but stream protocols such as TCP hide this from your
program.)

data structure
How your various pieces of data relate to each other and what shape they make when you
put them all together, as in a rectangular table or a triangular tree.

data type
A set of possible values, together with all the operations that know how to deal with
those values. For example, a numeric data type has a certain set of numbers that you can
work with, as well as various mathematical operations that you can do on the numbers, but
would make little sense on, say, a string such as "Kilroy". Strings have their own
operations, such as concatenation. Compound types made of a number of smaller pieces
generally have operations to compose and decompose them, and perhaps to rearrange them.
Objects that model things in the real world often have operations that correspond to real
activities. For instance, if you model an elevator, your elevator object might have an
"open_door" method.

DBM Stands for “Database Management” routines, a set of routines that emulate an associative
array using disk files. The routines use a dynamic hashing scheme to locate any entry
with only two disk accesses. DBM files allow a Perl program to keep a persistent hash
across multiple invocations. You can "tie" your hash variables to various DBM
implementations.

declaration
An assertion that states something exists and perhaps describes what it’s like, without
giving any commitment as to how or where you’ll use it. A declaration is like the part of
your recipe that says, “two cups flour, one large egg, four or five tadpoles…” See
statement for its opposite. Note that some declarations also function as statements.
Subroutine declarations also act as definitions if a body is supplied.

declarator
Something that tells your program what sort of variable you’d like. Perl doesn’t require
you to declare variables, but you can use "my", "our", or "state" to denote that you want
something other than the default.

decrement
To subtract a value from a variable, as in “decrement $x” (meaning to remove 1 from its
value) or “decrement $x by 3”.

default
A value chosen for you if you don’t supply a value of your own.

defined
Having a meaning. Perl thinks that some of the things people try to do are devoid of
meaning; in particular, making use of variables that have never been given a value and
performing certain operations on data that isn’t there. For example, if you try to read
data past the end of a file, Perl will hand you back an undefined value. See also false
and the "defined" entry in Camel chapter 27, “Functions”.

delimiter
A character or string that sets bounds to an arbitrarily sized textual object, not to be
confused with a separator or terminator. “To delimit” really just means “to surround” or
“to enclose” (like these parentheses are doing).

dereference
A fancy computer science term meaning “to follow a reference to what it points to”. The
“de” part of it refers to the fact that you’re taking away one level of indirection.

derived class
A class that defines some of its methods in terms of a more generic class, called a base
class. Note that classes aren’t classified exclusively into base classes or derived
classes: a class can function as both a derived class and a base class simultaneously,
which is kind of classy.

descriptor
See file descriptor.

destroy
To deallocate the memory of a referent (first triggering its "DESTROY" method, if it has
one).

destructor
A special method that is called when an object is thinking about destroying itself. A
Perl program’s "DESTROY" method doesn’t do the actual destruction; Perl just triggers the
method in case the class wants to do any associated cleanup.

device
A whiz-bang hardware gizmo (like a disk or tape drive or a modem or a joystick or a
mouse) attached to your computer, which the operating system tries to make look like a
file (or a bunch of files). Under Unix, these fake files tend to live in the /dev
directory.

directive
A pod directive. See Camel chapter 23, “Plain Old Documentation”.

directory
A special file that contains other files. Some operating systems call these “folders”,
“drawers”, “catalogues”, or “catalogs”.

directory handle
A name that represents a particular instance of opening a directory to read it, until you
close it. See the "opendir" function.

discipline
Some people need this and some people avoid it. For Perl, it’s an old way to say I/O
layer.

dispatch
To send something to its correct destination. Often used metaphorically to indicate a
transfer of programmatic control to a destination selected algorithmically, often by
lookup in a table of function references or, in the case of object methods, by traversing
the inheritance tree looking for the most specific definition for the method.

distribution
A standard, bundled release of a system of software. The default usage implies source
code is included. If that is not the case, it will be called a “binary-only”
distribution.

dual-lived
Some modules live both in the Standard Library and on CPAN. These modules might be
developed on two tracks as people modify either version. The trend currently is to
untangle these situations.

dweomer
An enchantment, illusion, phantasm, or jugglery. Said when Perl’s magical dwimmer effects
don’t do what you expect, but rather seem to be the product of arcane dweomercraft,
sorcery, or wonder working. [From Middle English.]

dwimmer
DWIM is an acronym for “Do What I Mean”, the principle that something should just do what
you want it to do without an undue amount of fuss. A bit of code that does “dwimming” is
a “dwimmer”. Dwimming can require a great deal of behind-the-scenes magic, which (if it
doesn’t stay properly behind the scenes) is called a dweomer instead.

dynamic scoping
Dynamic scoping works over a dynamic scope, making variables visible throughout the rest
of the block in which they are first used and in any subroutines that are called by the
rest of the block. Dynamically scoped variables can have their values temporarily changed
(and implicitly restored later) by a "local" operator. (Compare lexical scoping.) Used
more loosely to mean how a subroutine that is in the middle of calling another subroutine
“contains” that subroutine at runtime.

E
eclectic
Derived from many sources. Some would say too many.

element
A basic building block. When you’re talking about an array, it’s one of the items that
make up the array.

embedding
When something is contained in something else, particularly when that might be considered
surprising: “I’ve embedded a complete Perl interpreter in my editor!”

empty subclass test
The notion that an empty derived class should behave exactly like its base class.

encapsulation
The veil of abstraction separating the interface from the implementation (whether
enforced or not), which mandates that all access to an object’s state be through methods
alone.

endian
See little-endian and big-endian.

en passant
When you change a value as it is being copied. [From French “in passing”, as in the
exotic pawn-capturing maneuver in chess.]

environment
The collective set of environment variables your process inherits from its parent.
Accessed via %ENV.

environment variable
A mechanism by which some high-level agent such as a user can pass its preferences down
to its future offspring (child processes, grandchild processes, great-grandchild
processes, and so on). Each environment variable is a key/value pair, like one entry in a
hash.

EOF End of File. Sometimes used metaphorically as the terminating string of a here document.

errno
The error number returned by a syscall when it fails. Perl refers to the error by the
name $! (or $OS_ERROR if you use the English module).

error
See exception or fatal error.

escape sequence
See metasymbol.

exception
A fancy term for an error. See fatal error.

exception handling
The way a program responds to an error. The exception-handling mechanism in Perl is the
"eval" operator.

exec
To throw away the current process’s program and replace it with another, without exiting
the process or relinquishing any resources held (apart from the old memory image).

executable file
A file that is specially marked to tell the operating system that it’s okay to run this
file as a program. Usually shortened to “executable”.

execute
To run a program or subroutine. (Has nothing to do with the "kill" built-in, unless
you’re trying to run a signal handler.)

execute bit
The special mark that tells the operating system it can run this program. There are
actually three execute bits under Unix, and which bit gets used depends on whether you
own the file singularly, collectively, or not at all.

exit status
See status.

exploit
Used as a noun in this case, this refers to a known way to compromise a program to get it
to do something the author didn’t intend. Your task is to write unexploitable programs.

export
To make symbols from a module available for import by other modules.

expression
Anything you can legally say in a spot where a value is required. Typically composed of
literals, variables, operators, functions, and subroutine calls, not necessarily in that
order.

extension
A Perl module that also pulls in compiled C or C++ code. More generally, any experimental
option that can be compiled into Perl, such as multithreading.

F
false
In Perl, any value that would look like "" or "0" if evaluated in a string context. Since
undefined values evaluate to "", all undefined values are false, but not all false values
are undefined.

FAQ Frequently Asked Question (although not necessarily frequently answered, especially if
the answer appears in the Perl FAQ shipped standard with Perl).

fatal error
An uncaught exception, which causes termination of the process after printing a message
on your standard error stream. Errors that happen inside an "eval" are not fatal.
Instead, the "eval" terminates after placing the exception message in the $@
($EVAL_ERROR) variable. You can try to provoke a fatal error with the "die" operator
(known as throwing or raising an exception), but this may be caught by a dynamically
enclosing "eval". If not caught, the "die" becomes a fatal error.

feeping creaturism
A spoonerism of “creeping featurism”, noting the biological urge to add just one more
feature to a program.

field
A single piece of numeric or string data that is part of a longer string, record, or
line. Variable-width fields are usually split up by separators (so use "split" to extract
the fields), while fixed-width fields are usually at fixed positions (so use "unpack").
Instance variables are also known as “fields”.

FIFO
First In, First Out. See also LIFO. Also a nickname for a named pipe.

file
A named collection of data, usually stored on disk in a directory in a filesystem.
Roughly like a document, if you’re into office metaphors. In modern filesystems, you can
actually give a file more than one name. Some files have special properties, like
directories and devices.

file descriptor
The little number the operating system uses to keep track of which opened file you’re
talking about. Perl hides the file descriptor inside a standard I/O stream and then
attaches the stream to a filehandle.

fileglob
A “wildcard” match on filenames. See the "glob" function.

filehandle
An identifier (not necessarily related to the real name of a file) that represents a
particular instance of opening a file, until you close it. If you’re going to open and
close several different files in succession, it’s fine to open each of them with the same
filehandle, so you don’t have to write out separate code to process each file.

filename
One name for a file. This name is listed in a directory. You can use it in an "open" to
tell the operating system exactly which file you want to open, and associate the file
with a filehandle, which will carry the subsequent identity of that file in your program,
until you close it.

filesystem
A set of directories and files residing on a partition of the disk. Sometimes known as a
“partition”. You can change the file’s name or even move a file around from directory to
directory within a filesystem without actually moving the file itself, at least under
Unix.

file test operator
A built-in unary operator that you use to determine whether something is true about a
file, such as "–o $filename" to test whether you’re the owner of the file.

filter
A program designed to take a stream of input and transform it into a stream of output.

first-come
The first PAUSE author to upload a namespace automatically becomes the primary maintainer
for that namespace. The “first come” permissions distinguish a primary maintainer who was
assigned that role from one who received it automatically.

flag
We tend to avoid this term because it means so many things. It may mean a command-line
switch that takes no argument itself (such as Perl’s "–n" and "–p" flags) or, less
frequently, a single-bit indicator (such as the "O_CREAT" and "O_EXCL" flags used in
"sysopen"). Sometimes informally used to refer to certain regex modifiers.

floating point
A method of storing numbers in “scientific notation”, such that the precision of the
number is independent of its magnitude (the decimal point “floats”). Perl does its
numeric work with floating-point numbers (sometimes called “floats”) when it can’t get
away with using integers. Floating-point numbers are mere approximations of real numbers.

flush
The act of emptying a buffer, often before it’s full.

FMTEYEWTK
Far More Than Everything You Ever Wanted To Know. An exhaustive treatise on one narrow
topic, something of a super-FAQ. See Tom for far more.

foldcase
The casemap used in Unicode when comparing or matching without regard to case. Comparing
lower-, title-, or uppercase are all unreliable due to Unicode’s complex, one-to-many
case mappings. Foldcase is a lowercase variant (using a partially decomposed
normalization form for certain codepoints) created specifically to resolve this.

fork
To create a child process identical to the parent process at its moment of conception, at
least until it gets ideas of its own. A thread with protected memory.

formal arguments
The generic names by which a subroutine knows its arguments. In many languages, formal
arguments are always given individual names; in Perl, the formal arguments are just the
elements of an array. The formal arguments to a Perl program are $ARGV[0], $ARGV[1], and
so on. Similarly, the formal arguments to a Perl subroutine are $_[0], $_[1], and so on.
You may give the arguments individual names by assigning the values to a "my" list. See
also actual arguments.

format
A specification of how many spaces and digits and things to put somewhere so that
whatever you’re printing comes out nice and pretty.

freely available
Means you don’t have to pay money to get it, but the copyright on it may still belong to
someone else (like Larry).

freely redistributable
Means you’re not in legal trouble if you give a bootleg copy of it to your friends and we
find out about it. In fact, we’d rather you gave a copy to all your friends.

freeware
Historically, any software that you give away, particularly if you make the source code
available as well. Now often called open source software. Recently there has been a trend
to use the term in contradistinction to open source software, to refer only to free
software released under the Free Software Foundation’s GPL (General Public License), but
this is difficult to justify etymologically.

function
Mathematically, a mapping of each of a set of input values to a particular output value.
In computers, refers to a subroutine or operator that returns a value. It may or may not
have input values (called arguments).

funny character
Someone like Larry, or one of his peculiar friends. Also refers to the strange prefixes
that Perl requires as noun markers on its variables.

G
garbage collection
A misnamed feature—it should be called, “expecting your mother to pick up after you”.
Strictly speaking, Perl doesn’t do this, but it relies on a reference-counting mechanism
to keep things tidy. However, we rarely speak strictly and will often refer to the
reference-counting scheme as a form of garbage collection. (If it’s any comfort, when
your interpreter exits, a “real” garbage collector runs to make sure everything is
cleaned up if you’ve been messy with circular references and such.)

GID Group ID—in Unix, the numeric group ID that the operating system uses to identify you and
members of your group.

glob
Strictly, the shell’s "*" character, which will match a “glob” of characters when you’re
trying to generate a list of filenames. Loosely, the act of using globs and similar
symbols to do pattern matching. See also fileglob and typeglob.

global
Something you can see from anywhere, usually used of variables and subroutines that are
visible everywhere in your program. In Perl, only certain special variables are truly
global—most variables (and all subroutines) exist only in the current package. Global
variables can be declared with "our". See “Global Declarations” in Camel chapter 4,
“Statements and Declarations”.

global destruction
The garbage collection of globals (and the running of any associated object destructors)
that takes place when a Perl interpreter is being shut down. Global destruction should
not be confused with the Apocalypse, except perhaps when it should.

glue language
A language such as Perl that is good at hooking things together that weren’t intended to
be hooked together.

granularity
The size of the pieces you’re dealing with, mentally speaking.

grapheme
A graphene is an allotrope of carbon arranged in a hexagonal crystal lattice one atom
thick. A grapheme, or more fully, a grapheme cluster string is a single user-visible
character, which may in turn be several characters (codepoints) long. For example, a
carriage return plus a line feed is a single grapheme but two characters, while a “ȫ” is
a single grapheme but one, two, or even three characters, depending on normalization.

greedy
A subpattern whose quantifier wants to match as many things as possible.

grep
Originally from the old Unix editor command for “Globally search for a Regular Expression
and Print it”, now used in the general sense of any kind of search, especially text
searches. Perl has a built-in "grep" function that searches a list for elements matching
any given criterion, whereas the grep(1) program searches for lines matching a regular
expression in one or more files.

group
A set of users of which you are a member. In some operating systems (like Unix), you can
give certain file access permissions to other members of your group.

GV An internal “glob value” typedef, holding a typeglob. The "GV" type is a subclass of SV.

H
hacker
Someone who is brilliantly persistent in solving technical problems, whether these
involve golfing, fighting orcs, or programming. Hacker is a neutral term, morally
speaking. Good hackers are not to be confused with evil crackers or clueless script
kiddies. If you confuse them, we will presume that you are either evil or clueless.

handler
A subroutine or method that Perl calls when your program needs to respond to some
internal event, such as a signal, or an encounter with an operator subject to operator
overloading. See also callback.

hard reference
A scalar value containing the actual address of a referent, such that the referent’s
reference count accounts for it. (Some hard references are held internally, such as the
implicit reference from one of a typeglob’s variable slots to its corresponding
referent.) A hard reference is different from a symbolic reference.

hash
An unordered association of key/value pairs, stored such that you can easily use a string
key to look up its associated data value. This glossary is like a hash, where the word to
be defined is the key and the definition is the value. A hash is also sometimes
septisyllabically called an “associative array”, which is a pretty good reason for simply
calling it a “hash” instead.

hash table
A data structure used internally by Perl for implementing associative arrays (hashes)
efficiently. See also bucket.

header file
A file containing certain required definitions that you must include “ahead” of the rest
of your program to do certain obscure operations. A C header file has a .h extension.
Perl doesn’t really have header files, though historically Perl has sometimes used
translated .h files with a .ph extension. See "require" in Camel chapter 27, “Functions”.
(Header files have been superseded by the module mechanism.)

here document
So called because of a similar construct in shells that pretends that the lines following
the command are a separate file to be fed to the command, up to some terminating string.
In Perl, however, it’s just a fancy form of quoting.

hexadecimal
A number in base 16, “hex” for short. The digits for 10 through 15 are customarily
represented by the letters "a" through "f". Hexadecimal constants in Perl start with
"0x". See also the "hex" function in Camel chapter 27, “Functions”.

home directory
The directory you are put into when you log in. On a Unix system, the name is often
placed into $ENV{HOME} or $ENV{LOGDIR} by login, but you can also find it with
"(get""pwuid($<))[7]". (Some platforms do not have a concept of a home directory.)

host
The computer on which a program or other data resides.

hubris
Excessive pride, the sort of thing for which Zeus zaps you. Also the quality that makes
you write (and maintain) programs that other people won’t want to say bad things about.
Hence, the third great virtue of a programmer. See also laziness and impatience.

HV Short for a “hash value” typedef, which holds Perl’s internal representation of a hash.
The "HV" type is a subclass of SV.

I
identifier
A legally formed name for most anything in which a computer program might be interested.
Many languages (including Perl) allow identifiers to start with an alphabetic character,
and then contain alphabetics and digits. Perl also allows connector punctuation like the
underscore character wherever it allows alphabetics. (Perl also has more complicated
names, like qualified names.)

impatience
The anger you feel when the computer is being lazy. This makes you write programs that
don’t just react to your needs, but actually anticipate them. Or at least that pretend
to. Hence, the second great virtue of a programmer. See also laziness and hubris.

implementation
How a piece of code actually goes about doing its job. Users of the code should not count
on implementation details staying the same unless they are part of the published
interface.

import
To gain access to symbols that are exported from another module. See "use" in Camel
chapter 27, “Functions”.

increment
To increase the value of something by 1 (or by some other number, if so specified).

indexing
In olden days, the act of looking up a key in an actual index (such as a phone book). But
now it's merely the act of using any kind of key or position to find the corresponding
value, even if no index is involved. Things have degenerated to the point that Perl’s
"index" function merely locates the position (index) of one string in another.

indirect filehandle
An expression that evaluates to something that can be used as a filehandle: a string
(filehandle name), a typeglob, a typeglob reference, or a low-level IO object.

indirection
If something in a program isn’t the value you’re looking for but indicates where the
value is, that’s indirection. This can be done with either symbolic references or hard.

indirect object
In English grammar, a short noun phrase between a verb and its direct object indicating
the beneficiary or recipient of the action. In Perl, "print STDOUT "$foo\n";" can be
understood as “verb indirect-object object”, where "STDOUT" is the recipient of the
"print" action, and "$foo" is the object being printed. Similarly, when invoking a
method, you might place the invocant in the dative slot between the method and its
arguments:

$gollum = new Pathetic::Creature "Sméagol";
give $gollum "Fisssssh!";
give $gollum "Precious!";

indirect object slot
The syntactic position falling between a method call and its arguments when using the
indirect object invocation syntax. (The slot is distinguished by the absence of a comma
between it and the next argument.) "STDERR" is in the indirect object slot here:

print STDERR "Awake! Awake! Fear, Fire, Foes! Awake!\n";

infix
An operator that comes in between its operands, such as multiplication in "24 * 7".

inheritance
What you get from your ancestors, genetically or otherwise. If you happen to be a class,
your ancestors are called base classes and your descendants are called derived classes.
See single inheritance and multiple inheritance.

instance
Short for “an instance of a class”, meaning an object of that class.

instance data
See instance variable.

instance method
A method of an object, as opposed to a class method.

A method whose invocant is an object, not a package name. Every object of a class shares
all the methods of that class, so an instance method applies to all instances of the
class, rather than applying to a particular instance. Also see class method.

instance variable
An attribute of an object; data stored with the particular object rather than with the
class as a whole.

integer
A number with no fractional (decimal) part. A counting number, like 1, 2, 3, and so on,
but including 0 and the negatives.

interface
The services a piece of code promises to provide forever, in contrast to its
implementation, which it should feel free to change whenever it likes.

interpolation
The insertion of a scalar or list value somewhere in the middle of another value, such
that it appears to have been there all along. In Perl, variable interpolation happens in
double-quoted strings and patterns, and list interpolation occurs when constructing the
list of values to pass to a list operator or other such construct that takes a "LIST".

interpreter
Strictly speaking, a program that reads a second program and does what the second program
says directly without turning the program into a different form first, which is what
compilers do. Perl is not an interpreter by this definition, because it contains a kind
of compiler that takes a program and turns it into a more executable form (syntax trees)
within the perl process itself, which the Perl runtime system then interprets.

invocant
The agent on whose behalf a method is invoked. In a class method, the invocant is a
package name. In an instance method, the invocant is an object reference.

invocation
The act of calling up a deity, daemon, program, method, subroutine, or function to get it
to do what you think it’s supposed to do. We usually “call” subroutines but “invoke”
methods, since it sounds cooler.

I/O Input from, or output to, a file or device.

IO An internal I/O object. Can also mean indirect object.

I/O layer
One of the filters between the data and what you get as input or what you end up with as
output.

IPA India Pale Ale. Also the International Phonetic Alphabet, the standard alphabet used for
phonetic notation worldwide. Draws heavily on Unicode, including many combining
characters.

IP Internet Protocol, or Intellectual Property.

IPC Interprocess Communication.

is-a
A relationship between two objects in which one object is considered to be a more
specific version of the other, generic object: “A camel is a mammal.” Since the generic
object really only exists in a Platonic sense, we usually add a little abstraction to the
notion of objects and think of the relationship as being between a generic base class and
a specific derived class. Oddly enough, Platonic classes don’t always have Platonic
relationships—see inheritance.

iteration
Doing something repeatedly.

iterator
A special programming gizmo that keeps track of where you are in something that you’re
trying to iterate over. The "foreach" loop in Perl contains an iterator; so does a hash,
allowing you to "each" through it.

IV The integer four, not to be confused with six, Tom’s favorite editor. IV also means an
internal Integer Value of the type a scalar can hold, not to be confused with an NV.

J
JAPH
“Just Another Perl Hacker”, a clever but cryptic bit of Perl code that, when executed,
evaluates to that string. Often used to illustrate a particular Perl feature, and
something of an ongoing Obfuscated Perl Contest seen in USENET signatures.

K
key The string index to a hash, used to look up the value associated with that key.

keyword
See reserved words.

L
label
A name you give to a statement so that you can talk about that statement elsewhere in the
program.

laziness
The quality that makes you go to great effort to reduce overall energy expenditure. It
makes you write labor-saving programs that other people will find useful, and then
document what you wrote so you don’t have to answer so many questions about it. Hence,
the first great virtue of a programmer. Also hence, this book. See also impatience and
hubris.

leftmost longest
The preference of the regular expression engine to match the leftmost occurrence of a
pattern, then given a position at which a match will occur, the preference for the
longest match (presuming the use of a greedy quantifier). See Camel chapter 5, “Pattern
Matching” for much more on this subject.

left shift
A bit shift that multiplies the number by some power of 2.

lexeme
Fancy term for a token.

lexer
Fancy term for a tokener.

lexical analysis
Fancy term for tokenizing.

lexical scoping
Looking at your Oxford English Dictionary through a microscope. (Also known as static
scoping, because dictionaries don’t change very fast.) Similarly, looking at variables
stored in a private dictionary (namespace) for each scope, which are visible only from
their point of declaration down to the end of the lexical scope in which they are
declared. —Syn. static scoping. —Ant. dynamic scoping.

lexical variable
A variable subject to lexical scoping, declared by "my". Often just called a “lexical”.
(The "our" declaration declares a lexically scoped name for a global variable, which is
not itself a lexical variable.)

library
Generally, a collection of procedures. In ancient days, referred to a collection of
subroutines in a .pl file. In modern times, refers more often to the entire collection of
Perl modules on your system.

LIFO
Last In, First Out. See also FIFO. A LIFO is usually called a stack.

line
In Unix, a sequence of zero or more nonnewline characters terminated with a newline
character. On non-Unix machines, this is emulated by the C library even if the underlying
operating system has different ideas.

linebreak
A grapheme consisting of either a carriage return followed by a line feed or any
character with the Unicode Vertical Space character property.

line buffering
Used by a standard I/O output stream that flushes its buffer after every newline. Many
standard I/O libraries automatically set up line buffering on output that is going to the
terminal.

line number
The number of lines read previous to this one, plus 1. Perl keeps a separate line number
for each source or input file it opens. The current source file’s line number is
represented by "__LINE__". The current input line number (for the file that was most
recently read via "<FH>") is represented by the $. ($INPUT_LINE_NUMBER) variable. Many
error messages report both values, if available.

link
Used as a noun, a name in a directory that represents a file. A given file can have
multiple links to it. It’s like having the same phone number listed in the phone
directory under different names. As a verb, to resolve a partially compiled file’s
unresolved symbols into a (nearly) executable image. Linking can generally be static or
dynamic, which has nothing to do with static or dynamic scoping.

LIST
A syntactic construct representing a comma- separated list of expressions, evaluated to
produce a list value. Each expression in a "LIST" is evaluated in list context and
interpolated into the list value.

list
An ordered set of scalar values.

list context
The situation in which an expression is expected by its surroundings (the code calling
it) to return a list of values rather than a single value. Functions that want a "LIST"
of arguments tell those arguments that they should produce a list value. See also
context.

list operator
An operator that does something with a list of values, such as "join" or "grep". Usually
used for named built-in operators (such as "print", "unlink", and "system") that do not
require parentheses around their argument list.

list value
An unnamed list of temporary scalar values that may be passed around within a program
from any list-generating function to any function or construct that provides a list
context.

literal
A token in a programming language, such as a number or string, that gives you an actual
value instead of merely representing possible values as a variable does.

little-endian
From Swift: someone who eats eggs little end first. Also used of computers that store the
least significant byte of a word at a lower byte address than the most significant byte.
Often considered superior to big-endian machines. See also big-endian.

local
Not meaning the same thing everywhere. A global variable in Perl can be localized inside
a dynamic scope via the "local" operator.

logical operator
Symbols representing the concepts “and”, “or”, “xor”, and “not”.

lookahead
An assertion that peeks at the string to the right of the current match location.

lookbehind
An assertion that peeks at the string to the left of the current match location.

loop
A construct that performs something repeatedly, like a roller coaster.

loop control statement
Any statement within the body of a loop that can make a loop prematurely stop looping or
skip an iteration. Generally, you shouldn’t try this on roller coasters.

loop label
A kind of key or name attached to a loop (or roller coaster) so that loop control
statements can talk about which loop they want to control.

lowercase
In Unicode, not just characters with the General Category of Lowercase Letter, but any
character with the Lowercase property, including Modifier Letters, Letter Numbers, some
Other Symbols, and one Combining Mark.

lvaluable
Able to serve as an lvalue.

lvalue
Term used by language lawyers for a storage location you can assign a new value to, such
as a variable or an element of an array. The “l” is short for “left”, as in the left side
of an assignment, a typical place for lvalues. An lvaluable function or expression is one
to which a value may be assigned, as in "pos($x) = 10".

lvalue modifier
An adjectival pseudofunction that warps the meaning of an lvalue in some declarative
fashion. Currently there are three lvalue modifiers: "my", "our", and "local".

M
magic
Technically speaking, any extra semantics attached to a variable such as $!, $0, %ENV, or
%SIG, or to any tied variable. Magical things happen when you diddle those variables.

magical increment
An increment operator that knows how to bump up ASCII alphabetics as well as numbers.

magical variables
Special variables that have side effects when you access them or assign to them. For
example, in Perl, changing elements of the %ENV array also changes the corresponding
environment variables that subprocesses will use. Reading the $! variable gives you the
current system error number or message.

Makefile
A file that controls the compilation of a program. Perl programs don’t usually need a
Makefile because the Perl compiler has plenty of self-control.

man The Unix program that displays online documentation (manual pages) for you.

manpage
A “page” from the manuals, typically accessed via the man(1) command. A manpage contains
a SYNOPSIS, a DESCRIPTION, a list of BUGS, and so on, and is typically longer than a
page. There are manpages documenting commands, syscalls, library functions, devices,
protocols, files, and such. In this book, we call any piece of standard Perl
documentation (like perlop or perldelta) a manpage, no matter what format it’s installed
in on your system.

matching
See pattern matching.

member data
See instance variable.

memory
This always means your main memory, not your disk. Clouding the issue is the fact that
your machine may implement virtual memory; that is, it will pretend that it has more
memory than it really does, and it’ll use disk space to hold inactive bits. This can make
it seem like you have a little more memory than you really do, but it’s not a substitute
for real memory. The best thing that can be said about virtual memory is that it lets
your performance degrade gradually rather than suddenly when you run out of real memory.
But your program can die when you run out of virtual memory, too—if you haven’t thrashed
your disk to death first.

metacharacter
A character that is not supposed to be treated normally. Which characters are to be
treated specially as metacharacters varies greatly from context to context. Your shell
will have certain metacharacters, double-quoted Perl strings have other metacharacters,
and regular expression patterns have all the double-quote metacharacters plus some extra
ones of their own.

metasymbol
Something we’d call a metacharacter except that it’s a sequence of more than one
character. Generally, the first character in the sequence must be a true metacharacter
to get the other characters in the metasymbol to misbehave along with it.

method
A kind of action that an object can take if you tell it to. See Camel chapter 12,
“Objects”.

method resolution order
The path Perl takes through @INC. By default, this is a double depth first search, once
looking for defined methods and once for "AUTOLOAD". However, Perl lets you configure
this with "mro".

minicpan
A CPAN mirror that includes just the latest versions for each distribution, probably
created with "CPAN::Mini". See Camel chapter 19, “CPAN”.

minimalism
The belief that “small is beautiful”. Paradoxically, if you say something in a small
language, it turns out big, and if you say it in a big language, it turns out small. Go
figure.

mode
In the context of the stat(2) syscall, refers to the field holding the permission bits
and the type of the file.

modifier
See statement modifier, regular expression, and lvalue, not necessarily in that order.

module
A file that defines a package of (almost) the same name, which can either export symbols
or function as an object class. (A module’s main .pm file may also load in other files
in support of the module.) See the "use" built-in.

modulus
An integer divisor when you’re interested in the remainder instead of the quotient.

mojibake
When you speak one language and the computer thinks you’re speaking another. You’ll see
odd translations when you send UTF‑8, for instance, but the computer thinks you sent
Latin-1, showing all sorts of weird characters instead. The term is written 「文字化け」in
Japanese and means “character rot”, an apt description. Pronounced ["modʑibake"] in
standard IPA phonetics, or approximately “moh-jee-bah-keh”.

monger
Short for one member of Perl mongers, a purveyor of Perl.

mortal
A temporary value scheduled to die when the current statement finishes.

mro See method resolution order.

multidimensional array
An array with multiple subscripts for finding a single element. Perl implements these
using references—see Camel chapter 9, “Data Structures”.

multiple inheritance
The features you got from your mother and father, mixed together unpredictably. (See also
inheritance and single inheritance.) In computer languages (including Perl), it is the
notion that a given class may have multiple direct ancestors or base classes.

N
named pipe
A pipe with a name embedded in the filesystem so that it can be accessed by two unrelated
processes.

namespace
A domain of names. You needn’t worry about whether the names in one such domain have been
used in another. See package.

NaN Not a number. The value Perl uses for certain invalid or inexpressible floating-point
operations.

network address
The most important attribute of a socket, like your telephone’s telephone number.
Typically an IP address. See also port.

newline
A single character that represents the end of a line, with the ASCII value of 012 octal
under Unix (but 015 on a Mac), and represented by "\n" in Perl strings. For Windows
machines writing text files, and for certain physical devices like terminals, the single
newline gets automatically translated by your C library into a line feed and a carriage
return, but normally, no translation is done.

NFS Network File System, which allows you to mount a remote filesystem as if it were local.

normalization
Converting a text string into an alternate but equivalent canonical (or compatible)
representation that can then be compared for equivalence. Unicode recognizes four
different normalization forms: NFD, NFC, NFKD, and NFKC.

null character
A character with the numeric value of zero. It’s used by C to terminate strings, but Perl
allows strings to contain a null.

null list
A list value with zero elements, represented in Perl by "()".

null string
A string containing no characters, not to be confused with a string containing a null
character, which has a positive length and is true.

numeric context
The situation in which an expression is expected by its surroundings (the code calling
it) to return a number. See also context and string context.

numification
(Sometimes spelled nummification and nummify.) Perl lingo for implicit conversion into a
number; the related verb is numify. Numification is intended to rhyme with
mummification, and numify with mummify. It is unrelated to English numen, numina,
numinous. We originally forgot the extra m a long time ago, and some people got used to
our funny spelling, and so just as with "HTTP_REFERER"’s own missing letter, our weird
spelling has stuck around.

NV Short for Nevada, no part of which will ever be confused with civilization. NV also means
an internal floating- point Numeric Value of the type a scalar can hold, not to be
confused with an IV.

nybble
Half a byte, equivalent to one hexadecimal digit, and worth four bits.

O
object
An instance of a class. Something that “knows” what user-defined type (class) it is, and
what it can do because of what class it is. Your program can request an object to do
things, but the object gets to decide whether it wants to do them or not. Some objects
are more accommodating than others.

octal
A number in base 8. Only the digits 0 through 7 are allowed. Octal constants in Perl
start with 0, as in 013. See also the "oct" function.

offset
How many things you have to skip over when moving from the beginning of a string or array
to a specific position within it. Thus, the minimum offset is zero, not one, because you
don’t skip anything to get to the first item.

one-liner
An entire computer program crammed into one line of text.

open source software
Programs for which the source code is freely available and freely redistributable, with
no commercial strings attached. For a more detailed definition, see
<http://www.opensource.org/osd.html>.

operand
An expression that yields a value that an operator operates on. See also precedence.

operating system
A special program that runs on the bare machine and hides the gory details of managing
processes and devices. Usually used in a looser sense to indicate a particular culture
of programming. The loose sense can be used at varying levels of specificity. At one
extreme, you might say that all versions of Unix and Unix-lookalikes are the same
operating system (upsetting many people, especially lawyers and other advocates). At the
other extreme, you could say this particular version of this particular vendor’s
operating system is different from any other version of this or any other vendor’s
operating system. Perl is much more portable across operating systems than many other
languages. See also architecture and platform.

operator
A gizmo that transforms some number of input values to some number of output values,
often built into a language with a special syntax or symbol. A given operator may have
specific expectations about what types of data you give as its arguments (operands) and
what type of data you want back from it.

operator overloading
A kind of overloading that you can do on built-in operators to make them work on objects
as if the objects were ordinary scalar values, but with the actual semantics supplied by
the object class. This is set up with the overload pragma—see Camel chapter 13,
“Overloading”.

options
See either switches or regular expression modifiers.

ordinal
An abstract character’s integer value. Same thing as codepoint.

overloading
Giving additional meanings to a symbol or construct. Actually, all languages do
overloading to one extent or another, since people are good at figuring out things from
context.

overriding
Hiding or invalidating some other definition of the same name. (Not to be confused with
overloading, which adds definitions that must be disambiguated some other way.) To
confuse the issue further, we use the word with two overloaded definitions: to describe
how you can define your own subroutine to hide a built-in function of the same name (see
the section “Overriding Built-in Functions” in Camel chapter 11, “Modules”), and to
describe how you can define a replacement method in a derived class to hide a base
class’s method of the same name (see Camel chapter 12, “Objects”).

owner
The one user (apart from the superuser) who has absolute control over a file. A file may
also have a group of users who may exercise joint ownership if the real owner permits it.
See permission bits.

P
package
A namespace for global variables, subroutines, and the like, such that they can be kept
separate from like-named symbols in other namespaces. In a sense, only the package is
global, since the symbols in the package’s symbol table are only accessible from code
compiled outside the package by naming the package. But in another sense, all package
symbols are also globals—they’re just well-organized globals.

pad Short for scratchpad.

parameter
See argument.

parent class
See base class.

parse tree
See syntax tree.

parsing
The subtle but sometimes brutal art of attempting to turn your possibly malformed program
into a valid syntax tree.

patch
To fix by applying one, as it were. In the realm of hackerdom, a listing of the
differences between two versions of a program as might be applied by the patch(1) program
when you want to fix a bug or upgrade your old version.

PATH
The list of directories the system searches to find a program you want to execute. The
list is stored as one of your environment variables, accessible in Perl as $ENV{PATH}.

pathname
A fully qualified filename such as /usr/bin/perl. Sometimes confused with "PATH".

pattern
A template used in pattern matching.

pattern matching
Taking a pattern, usually a regular expression, and trying the pattern various ways on a
string to see whether there’s any way to make it fit. Often used to pick interesting
tidbits out of a file.

PAUSE
The Perl Authors Upload SErver (<http://pause.perl.org>), the gateway for modules on
their way to CPAN.

Perl mongers
A Perl user group, taking the form of its name from the New York Perl mongers, the first
Perl user group. Find one near you at <http://www.pm.org>.

permission bits
Bits that the owner of a file sets or unsets to allow or disallow access to other people.
These flag bits are part of the mode word returned by the "stat" built-in when you ask
about a file. On Unix systems, you can check the ls(1) manpage for more information.

Pern
What you get when you do "Perl++" twice. Doing it only once will curl your hair. You have
to increment it eight times to shampoo your hair. Lather, rinse, iterate.

pipe
A direct connection that carries the output of one process to the input of another
without an intermediate temporary file. Once the pipe is set up, the two processes in
question can read and write as if they were talking to a normal file, with some caveats.

pipeline
A series of processes all in a row, linked by pipes, where each passes its output stream
to the next.

platform
The entire hardware and software context in which a program runs. A program written in a
platform-dependent language might break if you change any of the following: machine,
operating system, libraries, compiler, or system configuration. The perl interpreter has
to be compiled differently for each platform because it is implemented in C, but programs
written in the Perl language are largely platform independent.

pod The markup used to embed documentation into your Perl code. Pod stands for “Plain old
documentation”. See Camel chapter 23, “Plain Old Documentation”.

pod command
A sequence, such as "=head1", that denotes the start of a pod section.

pointer
A variable in a language like C that contains the exact memory location of some other
item. Perl handles pointers internally so you don’t have to worry about them. Instead,
you just use symbolic pointers in the form of keys and variable names, or hard
references, which aren’t pointers (but act like pointers and do in fact contain
pointers).

polymorphism
The notion that you can tell an object to do something generic, and the object will
interpret the command in different ways depending on its type. [< Greek πολυ- + μορϕή,
many forms.]

port
The part of the address of a TCP or UDP socket that directs packets to the correct
process after finding the right machine, something like the phone extension you give when
you reach the company operator. Also the result of converting code to run on a different
platform than originally intended, or the verb denoting this conversion.

portable
Once upon a time, C code compilable under both BSD and SysV. In general, code that can be
easily converted to run on another platform, where “easily” can be defined however you
like, and usually is. Anything may be considered portable if you try hard enough, such
as a mobile home or London Bridge.

porter
Someone who “carries” software from one platform to another. Porting programs written in
platform-dependent languages such as C can be difficult work, but porting programs like
Perl is very much worth the agony.

possessive
Said of quantifiers and groups in patterns that refuse to give up anything once they’ve
gotten their mitts on it. Catchier and easier to say than the even more formal
nonbacktrackable.

POSIX
The Portable Operating System Interface specification.

postfix
An operator that follows its operand, as in "$x++".

pp An internal shorthand for a “push- pop” code; that is, C code implementing Perl’s stack
machine.

pragma
A standard module whose practical hints and suggestions are received (and possibly
ignored) at compile time. Pragmas are named in all lowercase.

precedence
The rules of conduct that, in the absence of other guidance, determine what should happen
first. For example, in the absence of parentheses, you always do multiplication before
addition.

prefix
An operator that precedes its operand, as in "++$x".

preprocessing
What some helper process did to transform the incoming data into a form more suitable for
the current process. Often done with an incoming pipe. See also C preprocessor.

primary maintainer
The author that PAUSE allows to assign co-maintainer permissions to a namespace. A
primary maintainer can give up this distinction by assigning it to another PAUSE author.
See Camel chapter 19, “CPAN”.

procedure
A subroutine.

process
An instance of a running program. Under multitasking systems like Unix, two or more
separate processes could be running the same program independently at the same time—in
fact, the "fork" function is designed to bring about this happy state of affairs. Under
other operating systems, processes are sometimes called “threads”, “tasks”, or “jobs”,
often with slight nuances in meaning.

program
See script.

program generator
A system that algorithmically writes code for you in a high-level language. See also code
generator.

progressive matching
Pattern matching matching>that picks up where it left off before.

property
See either instance variable or character property.

protocol
In networking, an agreed-upon way of sending messages back and forth so that neither
correspondent will get too confused.

prototype
An optional part of a subroutine declaration telling the Perl compiler how many and what
flavor of arguments may be passed as actual arguments, so you can write subroutine calls
that parse much like built-in functions. (Or don’t parse, as the case may be.)

pseudofunction
A construct that sometimes looks like a function but really isn’t. Usually reserved for
lvalue modifiers like "my", for context modifiers like "scalar", and for the pick-your-
own-quotes constructs, "q//", "qq//", "qx//", "qw//", "qr//", "m//", "s///", "y///", and
"tr///".

pseudohash
Formerly, a reference to an array whose initial element happens to hold a reference to a
hash. You used to be able to treat a pseudohash reference as either an array reference or
a hash reference. Pseudohashes are no longer supported.

pseudoliteral
An operator X"that looks something like a literal, such as the output-grabbing operator,
<literal moreinfo="none""`>"command""`".

public domain
Something not owned by anybody. Perl is copyrighted and is thus not in the public domain—
it’s just freely available and freely redistributable.

pumpkin
A notional “baton” handed around the Perl community indicating who is the lead integrator
in some arena of development.

pumpking
A pumpkin holder, the person in charge of pumping the pump, or at least priming it. Must
be willing to play the part of the Great Pumpkin now and then.

PV A “pointer value”, which is Perl Internals Talk for a "char*".

Q
qualified
Possessing a complete name. The symbol $Ent::moot is qualified; $moot is unqualified. A
fully qualified filename is specified from the top-level directory.

quantifier
A component of a regular expression specifying how many times the foregoing atom may
occur.

R
race condition
A race condition exists when the result of several interrelated events depends on the
ordering of those events, but that order cannot be guaranteed due to nondeterministic
timing effects. If two or more programs, or parts of the same program, try to go through
the same series of events, one might interrupt the work of the other. This is a good way
to find an exploit.

readable
With respect to files, one that has the proper permission bit set to let you access the
file. With respect to computer programs, one that’s written well enough that someone has
a chance of figuring out what it’s trying to do.

reaping
The last rites performed by a parent process on behalf of a deceased child process so
that it doesn’t remain a zombie. See the "wait" and "waitpid" function calls.

record
A set of related data values in a file or stream, often associated with a unique key
field. In Unix, often commensurate with a line, or a blank-line–terminated set of lines
(a “paragraph”). Each line of the /etc/passwd file is a record, keyed on login name,
containing information about that user.

recursion
The art of defining something (at least partly) in terms of itself, which is a naughty
no-no in dictionaries but often works out okay in computer programs if you’re careful not
to recurse forever (which is like an infinite loop with more spectacular failure modes).

reference
Where you look to find a pointer to information somewhere else. (See indirection.)
References come in two flavors: symbolic references and hard references.

referent
Whatever a reference refers to, which may or may not have a name. Common types of
referents include scalars, arrays, hashes, and subroutines.

regex
See regular expression.

regular expression
A single entity with various interpretations, like an elephant. To a computer scientist,
it’s a grammar for a little language in which some strings are legal and others aren’t.
To normal people, it’s a pattern you can use to find what you’re looking for when it
varies from case to case. Perl’s regular expressions are far from regular in the
theoretical sense, but in regular use they work quite well. Here’s a regular expression:
"/Oh s.*t./". This will match strings like “"Oh say can you see by the dawn's early
light"” and “"Oh sit!"”. See Camel chapter 5, “Pattern Matching”.

regular expression modifier
An option on a pattern or substitution, such as "/i" to render the pattern case-
insensitive.

regular file
A file that’s not a directory, a device, a named pipe or socket, or a symbolic link. Perl
uses the "–f" file test operator to identify regular files. Sometimes called a “plain”
file.

relational operator
An operator that says whether a particular ordering relationship is true about a pair of
operands. Perl has both numeric and string relational operators. See collating sequence.

reserved words
A word with a specific, built-in meaning to a compiler, such as "if" or "delete". In many
languages (not Perl), it’s illegal to use reserved words to name anything else. (Which is
why they’re reserved, after all.) In Perl, you just can’t use them to name labels or
filehandles. Also called “keywords”.

return value
The value produced by a subroutine or expression when evaluated. In Perl, a return value
may be either a list or a scalar.

RFC Request For Comment, which despite the timid connotations is the name of a series of
important standards documents.

right shift
A bit shift that divides a number by some power of 2.

role
A name for a concrete set of behaviors. A role is a way to add behavior to a class
without inheritance.

root
The superuser ("UID" == 0). Also the top-level directory of the filesystem.

RTFM
What you are told when someone thinks you should Read The Fine Manual.

run phase
Any time after Perl starts running your main program. See also compile phase. Run phase
is mostly spent in runtime but may also be spent in compile time when "require", "do"
"FILE", or "eval" "STRING" operators are executed, or when a substitution uses the "/ee"
modifier.

runtime
The time when Perl is actually doing what your code says to do, as opposed to the earlier
period of time when it was trying to figure out whether what you said made any sense
whatsoever, which is compile time.

runtime pattern
A pattern that contains one or more variables to be interpolated before parsing the
pattern as a regular expression, and that therefore cannot be analyzed at compile time,
but must be reanalyzed each time the pattern match operator is evaluated. Runtime
patterns are useful but expensive.

RV A recreational vehicle, not to be confused with vehicular recreation. RV also means an
internal Reference Value of the type a scalar can hold. See also IV and NV if you’re not
confused yet.

rvalue
A value that you might find on the right side of an assignment. See also lvalue.

S
sandbox
A walled off area that’s not supposed to affect beyond its walls. You let kids play in
the sandbox instead of running in the road. See Camel chapter 20, “Security”.

scalar
A simple, singular value; a number, string, or reference.

scalar context
The situation in which an expression is expected by its surroundings (the code calling
it) to return a single value rather than a list of values. See also context and list
context. A scalar context sometimes imposes additional constraints on the return value—
see string context and numeric context. Sometimes we talk about a Boolean context inside
conditionals, but this imposes no additional constraints, since any scalar value, whether
numeric or string, is already true or false.

scalar literal
A number or quoted string—an actual value in the text of your program, as opposed to a
variable.

scalar value
A value that happens to be a scalar as opposed to a list.

scalar variable
A variable prefixed with "$" that holds a single value.

scope
From how far away you can see a variable, looking through one. Perl has two visibility
mechanisms. It does dynamic scoping of "local" variables, meaning that the rest of the
block, and any subroutines that are called by the rest of the block, can see the
variables that are local to the block. Perl does lexical scoping of "my" variables,
meaning that the rest of the block can see the variable, but other subroutines called by
the block cannot see the variable.

scratchpad
The area in which a particular invocation of a particular file or subroutine keeps some
of its temporary values, including any lexically scoped variables.

script
A text file that is a program intended to be executed directly rather than compiled to
another form of file before execution.

Also, in the context of Unicode, a writing system for a particular language or group of
languages, such as Greek, Bengali, or Tengwar.

script kiddie
A cracker who is not a hacker but knows just enough to run canned scripts. A cargo-cult
programmer.

sed A venerable Stream EDitor from which Perl derives some of its ideas.

semaphore
A fancy kind of interlock that prevents multiple threads or processes from using up the
same resources simultaneously.

separator
A character or string that keeps two surrounding strings from being confused with each
other. The "split" function works on separators. Not to be confused with delimiters or
terminators. The “or” in the previous sentence separated the two alternatives.

serialization
Putting a fancy data structure into linear order so that it can be stored as a string in
a disk file or database, or sent through a pipe. Also called marshalling.

server
In networking, a process that either advertises a service or just hangs around at a known
location and waits for clients who need service to get in touch with it.

service
Something you do for someone else to make them happy, like giving them the time of day
(or of their life). On some machines, well-known services are listed by the "getservent"
function.

setgid
Same as setuid, only having to do with giving away group privileges.

setuid
Said of a program that runs with the privileges of its owner rather than (as is usually
the case) the privileges of whoever is running it. Also describes the bit in the mode
word (permission bits) that controls the feature. This bit must be explicitly set by the
owner to enable this feature, and the program must be carefully written not to give away
more privileges than it ought to.

shared memory
A piece of memory accessible by two different processes who otherwise would not see each
other’s memory.

shebang
Irish for the whole McGillicuddy. In Perl culture, a portmanteau of “sharp” and “bang”,
meaning the "#!" sequence that tells the system where to find the interpreter.

shell
A command-line interpreter. The program that interactively gives you a prompt, accepts
one or more lines of input, and executes the programs you mentioned, feeding each of them
their proper arguments and input data. Shells can also execute scripts containing such
commands. Under Unix, typical shells include the Bourne shell (/bin/sh), the C shell
(/bin/csh), and the Korn shell (/bin/ksh). Perl is not strictly a shell because it’s not
interactive (although Perl programs can be interactive).

side effects
Something extra that happens when you evaluate an expression. Nowadays it can refer to
almost anything. For example, evaluating a simple assignment statement typically has the
“side effect” of assigning a value to a variable. (And you thought assigning the value
was your primary intent in the first place!) Likewise, assigning a value to the special
variable $| ($AUTOFLUSH) has the side effect of forcing a flush after every "write" or
"print" on the currently selected filehandle.

sigil
A glyph used in magic. Or, for Perl, the symbol in front of a variable name, such as "$",
"@", and "%".

signal
A bolt out of the blue; that is, an event triggered by the operating system, probably
when you’re least expecting it.

signal handler
A subroutine that, instead of being content to be called in the normal fashion, sits
around waiting for a bolt out of the blue before it will deign to execute. Under Perl,
bolts out of the blue are called signals, and you send them with the "kill" built-in. See
the %SIG hash in Camel chapter 25, “Special Names” and the section “Signals” in Camel
chapter 15, “Interprocess Communication”.

single inheritance
The features you got from your mother, if she told you that you don’t have a father. (See
also inheritance and multiple inheritance.) In computer languages, the idea that classes
reproduce asexually so that a given class can only have one direct ancestor or base
class. Perl supplies no such restriction, though you may certainly program Perl that way
if you like.

slice
A selection of any number of elements from a list, array, or hash.

slurp
To read an entire file into a string in one operation.

socket
An endpoint for network communication among multiple processes that works much like a
telephone or a post office box. The most important thing about a socket is its network
address (like a phone number). Different kinds of sockets have different kinds of
addresses—some look like filenames, and some don’t.

soft reference
See symbolic reference.

source filter
A special kind of module that does preprocessing on your script just before it gets to
the tokener.

stack
A device you can put things on the top of, and later take them back off in the opposite
order in which you put them on. See LIFO.

standard
Included in the official Perl distribution, as in a standard module, a standard tool, or
a standard Perl manpage.

standard error
The default output stream for nasty remarks that don’t belong in standard output.
Represented within a Perl program by the output> filehandle "STDERR". You can use this
stream explicitly, but the "die" and "warn" built-ins write to your standard error stream
automatically (unless trapped or otherwise intercepted).

standard input
The default input stream for your program, which if possible shouldn’t care where its
data is coming from. Represented within a Perl program by the filehandle "STDIN".

standard I/O
A standard C library for doing buffered input and output to the operating system. (The
“standard” of standard I/O is at most marginally related to the “standard” of standard
input and output.) In general, Perl relies on whatever implementation of standard I/O a
given operating system supplies, so the buffering characteristics of a Perl program on
one machine may not exactly match those on another machine. Normally this only
influences efficiency, not semantics. If your standard I/O package is doing block
buffering and you want it to flush the buffer more often, just set the $| variable to a
true value.

Standard Library
Everything that comes with the official perl distribution. Some vendor versions of perl
change their distributions, leaving out some parts or including extras. See also dual-
lived.

standard output
The default output stream for your program, which if possible shouldn’t care where its
data is going. Represented within a Perl program by the filehandle "STDOUT".

statement
A command to the computer about what to do next, like a step in a recipe: “Add marmalade
to batter and mix until mixed.” A statement is distinguished from a declaration, which
doesn’t tell the computer to do anything, but just to learn something.

statement modifier
A conditional or loop that you put after the statement instead of before, if you know
what we mean.

static
Varying slowly compared to something else. (Unfortunately, everything is relatively
stable compared to something else, except for certain elementary particles, and we’re not
so sure about them.) In computers, where things are supposed to vary rapidly, “static”
has a derogatory connotation, indicating a slightly dysfunctional variable, subroutine,
or method. In Perl culture, the word is politely avoided.

If you’re a C or C++ programmer, you might be looking for Perl’s "state" keyword.

static method
No such thing. See class method.

static scoping
No such thing. See lexical scoping.

static variable
No such thing. Just use a lexical variable in a scope larger than your subroutine, or
declare it with "state" instead of with "my".

stat structure
A special internal spot in which Perl keeps the information about the last file on which
you requested information.

status
The value returned to the parent process when one of its child processes dies. This value
is placed in the special variable $?. Its upper eight bits are the exit status of the
defunct process, and its lower eight bits identify the signal (if any) that the process
died from. On Unix systems, this status value is the same as the status word returned by
wait(2). See "system" in Camel chapter 27, “Functions”.

STDERR
See standard error.

STDIN
See standard input.

STDIO
See standard I/O.

STDOUT
See standard output.

stream
A flow of data into or out of a process as a steady sequence of bytes or characters,
without the appearance of being broken up into packets. This is a kind of interface—the
underlying implementation may well break your data up into separate packets for delivery,
but this is hidden from you.

string
A sequence of characters such as “He said !@#*&%@#*?!”. A string does not have to be
entirely printable.

string context
The situation in which an expression is expected by its surroundings (the code calling
it) to return a string. See also context and numeric context.

stringification
The process of producing a string representation of an abstract object.

struct
C keyword introducing a structure definition or name.

structure
See data structure.

subclass
See derived class.

subpattern
A component of a regular expression pattern.

subroutine
A named or otherwise accessible piece of program that can be invoked from elsewhere in
the program in order to accomplish some subgoal of the program. A subroutine is often
parameterized to accomplish different but related things depending on its input
arguments. If the subroutine returns a meaningful value, it is also called a function.

subscript
A value that indicates the position of a particular array element in an array.

substitution
Changing parts of a string via the "s///" operator. (We avoid use of this term to mean
variable interpolation.)

substring
A portion of a string, starting at a certain character position (offset) and proceeding
for a certain number of characters.

superclass
See base class.

superuser
The person whom the operating system will let do almost anything. Typically your system
administrator or someone pretending to be your system administrator. On Unix systems, the
root user. On Windows systems, usually the Administrator user.

SV Short for “scalar value”. But within the Perl interpreter, every referent is treated as a
member of a class derived from SV, in an object-oriented sort of way. Every value inside
Perl is passed around as a C language "SV*" pointer. The SV struct knows its own
“referent type”, and the code is smart enough (we hope) not to try to call a hash
function on a subroutine.

switch
An option you give on a command line to influence the way your program works, usually
introduced with a minus sign. The word is also used as a nickname for a switch
statement.

switch cluster
The combination of multiple command- line switches (e.g., "–a –b –c") into one switch
(e.g., "–abc"). Any switch with an additional argument must be the last switch in a
cluster.

switch statement
A program technique that lets you evaluate an expression and then, based on the value of
the expression, do a multiway branch to the appropriate piece of code for that value.
Also called a “case structure”, named after the similar Pascal construct. Most switch
statements in Perl are spelled "given". See “The "given" statement” in Camel chapter 4,
“Statements and Declarations”.

symbol
Generally, any token or metasymbol. Often used more specifically to mean the sort of name
you might find in a symbol table.

symbolic debugger
A program that lets you step through the execution of your program, stopping or printing
things out here and there to see whether anything has gone wrong, and, if so, what. The
“symbolic” part just means that you can talk to the debugger using the same symbols with
which your program is written.

symbolic link
An alternate filename that points to the real filename, which in turn points to the real
file. Whenever the operating system is trying to parse a pathname containing a symbolic
link, it merely substitutes the new name and continues parsing.

symbolic reference
A variable whose value is the name of another variable or subroutine. By dereferencing
the first variable, you can get at the second one. Symbolic references are illegal under
"use strict "refs"".

symbol table
Where a compiler remembers symbols. A program like Perl must somehow remember all the
names of all the variables, filehandles, and subroutines you’ve used. It does this by
placing the names in a symbol table, which is implemented in Perl using a hash table.
There is a separate symbol table for each package to give each package its own namespace.

synchronous
Programming in which the orderly sequence of events can be determined; that is, when
things happen one after the other, not at the same time.

syntactic sugar
An alternative way of writing something more easily; a shortcut.

syntax
From Greek σύνταξις, “with-arrangement”. How things (particularly symbols) are put
together with each other.

syntax tree
An internal representation of your program wherein lower-level constructs dangle off the
higher-level constructs enclosing them.

syscall
A function call directly to the operating system. Many of the important subroutines and
functions you use aren’t direct system calls, but are built up in one or more layers
above the system call level. In general, Perl programmers don’t need to worry about the
distinction. However, if you do happen to know which Perl functions are really syscalls,
you can predict which of these will set the $! ($ERRNO) variable on failure.
Unfortunately, beginning programmers often confusingly employ the term “system call” to
mean what happens when you call the Perl "system" function, which actually involves many
syscalls. To avoid any confusion, we nearly always say “syscall” for something you could
call indirectly via Perl’s "syscall" function, and never for something you would call
with Perl’s "system" function.

T
taint checks
The special bookkeeping Perl does to track the flow of external data through your program
and disallow their use in system commands.

tainted
Said of data derived from the grubby hands of a user, and thus unsafe for a secure
program to rely on. Perl does taint checks if you run a setuid (or setgid) program, or if
you use the "–T" switch.

taint mode
Running under the "–T" switch, marking all external data as suspect and refusing to use
it with system commands. See Camel chapter 20, “Security”.

TCP Short for Transmission Control Protocol. A protocol wrapped around the Internet Protocol
to make an unreliable packet transmission mechanism appear to the application program to
be a reliable stream of bytes. (Usually.)

term
Short for a “terminal”—that is, a leaf node of a syntax tree. A thing that functions
grammatically as an operand for the operators in an expression.

terminator
A character or string that marks the end of another string. The $/ variable contains the
string that terminates a "readline" operation, which "chomp" deletes from the end. Not to
be confused with delimiters or separators. The period at the end of this sentence is a
terminator.

ternary
An operator taking three operands. Sometimes pronounced trinary.

text
A string or file containing primarily printable characters.

thread
Like a forked process, but without fork’s inherent memory protection. A thread is lighter
weight than a full process, in that a process could have multiple threads running around
in it, all fighting over the same process’s memory space unless steps are taken to
protect threads from one another.

tie The bond between a magical variable and its implementation class. See the "tie" function
in Camel chapter 27, “Functions” and Camel chapter 14, “Tied Variables”.

titlecase
The case used for capitals that are followed by lowercase characters instead of by more
capitals. Sometimes called sentence case or headline case. English doesn’t use Unicode
titlecase, but casing rules for English titles are more complicated than simply
capitalizing each word’s first character.

TMTOWTDI
There’s More Than One Way To Do It, the Perl Motto. The notion that there can be more
than one valid path to solving a programming problem in context. (This doesn’t mean that
more ways are always better or that all possible paths are equally desirable—just that
there need not be One True Way.)

token
A morpheme in a programming language, the smallest unit of text with semantic
significance.

tokener
A module that breaks a program text into a sequence of tokens for later analysis by a
parser.

tokenizing
Splitting up a program text into tokens. Also known as “lexing”, in which case you get
“lexemes” instead of tokens.

toolbox approach
The notion that, with a complete set of simple tools that work well together, you can
build almost anything you want. Which is fine if you’re assembling a tricycle, but if
you’re building a defranishizing comboflux regurgalator, you really want your own machine
shop in which to build special tools. Perl is sort of a machine shop.

topic
The thing you’re working on. Structures like "while(<>)", "for", "foreach", and "given"
set the topic for you by assigning to $_, the default (topic) variable.

transliterate
To turn one string representation into another by mapping each character of the source
string to its corresponding character in the result string. Not to be confused with
translation: for example, Greek πολύχρωμος transliterates into polychromos but translates
into many-colored. See the "tr///" operator in Camel chapter 5, “Pattern Matching”.

trigger
An event that causes a handler to be run.

trinary
Not a stellar system with three stars, but an operator taking three operands. Sometimes
pronounced ternary.

troff
A venerable typesetting language from which Perl derives the name of its $% variable and
which is secretly used in the production of Camel books.

true
Any scalar value that doesn’t evaluate to 0 or "".

truncating
Emptying a file of existing contents, either automatically when opening a file for
writing or explicitly via the "truncate" function.

type
See data type and class.

type casting
Converting data from one type to another. C permits this. Perl does not need it. Nor
want it.

typedef
A type definition in the C and C++ languages.

typed lexical
A lexical variable lexical>that is declared with a class type: "my Pony $bill".

typeglob
Use of a single identifier, prefixed with "*". For example, *name stands for any or all
of $name, @name, %name, &name, or just "name". How you use it determines whether it is
interpreted as all or only one of them. See “Typeglobs and Filehandles” in Camel chapter
2, “Bits and Pieces”.

typemap
A description of how C types may be transformed to and from Perl types within an
extension module written in XS.

U
UDP User Datagram Protocol, the typical way to send datagrams over the Internet.

UID A user ID. Often used in the context of file or process ownership.

umask
A mask of those permission bits that should be forced off when creating files or
directories, in order to establish a policy of whom you’ll ordinarily deny access to. See
the "umask" function.

unary operator
An operator with only one operand, like "!" or "chdir". Unary operators are usually
prefix operators; that is, they precede their operand. The "++" and "––" operators can be
either prefix or postfix. (Their position does change their meanings.)

Unicode
A character set comprising all the major character sets of the world, more or less. See
<http://www.unicode.org>.

Unix
A very large and constantly evolving language with several alternative and largely
incompatible syntaxes, in which anyone can define anything any way they choose, and
usually do. Speakers of this language think it’s easy to learn because it’s so easily
twisted to one’s own ends, but dialectical differences make tribal intercommunication
nearly impossible, and travelers are often reduced to a pidgin-like subset of the
language. To be universally understood, a Unix shell programmer must spend years of study
in the art. Many have abandoned this discipline and now communicate via an Esperanto-like
language called Perl.

In ancient times, Unix was also used to refer to some code that a couple of people at
Bell Labs wrote to make use of a PDP-7 computer that wasn’t doing much of anything else
at the time.

uppercase
In Unicode, not just characters with the General Category of Uppercase Letter, but any
character with the Uppercase property, including some Letter Numbers and Symbols. Not to
be confused with titlecase.

V
value
An actual piece of data, in contrast to all the variables, references, keys, indices,
operators, and whatnot that you need to access the value.

variable
A named storage location that can hold any of various kinds of value, as your program
sees fit.

variable interpolation
The interpolation of a scalar or array variable into a string.

variadic
Said of a function that happily receives an indeterminate number of actual arguments.

vector
Mathematical jargon for a list of scalar values.

virtual
Providing the appearance of something without the reality, as in: virtual memory is not
real memory. (See also memory.) The opposite of “virtual” is “transparent”, which means
providing the reality of something without the appearance, as in: Perl handles the
variable-length UTF‑8 character encoding transparently.

void context
A form of scalar context in which an expression is not expected to return any value at
all and is evaluated for its side effects alone.

v-string
A “version” or “vector” string specified with a "v" followed by a series of decimal
integers in dot notation, for instance, "v1.20.300.4000". Each number turns into a
character with the specified ordinal value. (The "v" is optional when there are at least
three integers.)

W
warning
A message printed to the "STDERR" stream to the effect that something might be wrong but
isn’t worth blowing up over. See "warn" in Camel chapter 27, “Functions” and the
"warnings" pragma in Camel chapter 28, “Pragmantic Modules”.

watch expression
An expression which, when its value changes, causes a breakpoint in the Perl debugger.

weak reference
A reference that doesn’t get counted normally. When all the normal references to data
disappear, the data disappears. These are useful for circular references that would never
disappear otherwise.

whitespace
A character that moves your cursor but doesn’t otherwise put anything on your screen.
Typically refers to any of: space, tab, line feed, carriage return, or form feed. In
Unicode, matches many other characters that Unicode considers whitespace, including the
ɴ-ʙʀ .

word
In normal “computerese”, the piece of data of the size most efficiently handled by your
computer, typically 32 bits or so, give or take a few powers of 2. In Perl culture, it
more often refers to an alphanumeric identifier (including underscores), or to a string
of nonwhitespace characters bounded by whitespace or string boundaries.

working directory
Your current directory, from which relative pathnames are interpreted by the operating
system. The operating system knows your current directory because you told it with a
"chdir", or because you started out in the place where your parent process was when you
were born.

wrapper
A program or subroutine that runs some other program or subroutine for you, modifying
some of its input or output to better suit your purposes.

WYSIWYG
What You See Is What You Get. Usually used when something that appears on the screen
matches how it will eventually look, like Perl’s "format" declarations. Also used to mean
the opposite of magic because everything works exactly as it appears, as in the three-
argument form of "open".

X
XS An extraordinarily exported, expeditiously excellent, expressly eXternal Subroutine,
executed in existing C or C++ or in an exciting extension language called
(exasperatingly) XS.

XSUB
An external subroutine defined in XS.

Y
yacc
Yet Another Compiler Compiler. A parser generator without which Perl probably would not
have existed. See the file perly.y in the Perl source distribution.

Z
zero width
A subpattern assertion matching the null string between characters.

zombie
A process that has died (exited) but whose parent has not yet received proper
notification of its demise by virtue of having called "wait" or "waitpid". If you "fork",
you must clean up after your child processes when they exit; otherwise, the process table
will fill up and your system administrator will Not Be Happy with you.

AUTHOR AND COPYRIGHT
Based on the Glossary of Programming Perl, Fourth Edition, by Tom Christiansen, brian d foy,
Larry Wall, & Jon Orwant. Copyright (c) 2000, 1996, 1991, 2012 O'Reilly Media, Inc. This
document may be distributed under the same terms as Perl itself.

perl v5.34.0 2025-07-25 PERLGLOSSARY(1)

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