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PERLINTRO(1)                      Perl Programmers Reference Guide                      PERLINTRO(1)



NAME
       perlintro - a brief introduction and overview of Perl

DESCRIPTION
       This document is intended to give you a quick overview of the Perl programming language,
       along with pointers to further documentation.  It is intended as a "bootstrap" guide for
       those who are new to the language, and provides just enough information for you to be able to
       read other peoples' Perl and understand roughly what it's doing, or write your own simple
       scripts.

       This introductory document does not aim to be complete.  It does not even aim to be entirely
       accurate.  In some cases perfection has been sacrificed in the goal of getting the general
       idea across.  You are strongly advised to follow this introduction with more information from
       the full Perl manual, the table of contents to which can be found in perltoc.

       Throughout this document you'll see references to other parts of the Perl documentation.  You
       can read that documentation using the "perldoc" command or whatever method you're using to
       read this document.

       Throughout Perl's documentation, you'll find numerous examples intended to help explain the
       discussed features.  Please keep in mind that many of them are code fragments rather than
       complete programs.

       These examples often reflect the style and preference of the author of that piece of the
       documentation, and may be briefer than a corresponding line of code in a real program.
       Except where otherwise noted, you should assume that "use strict" and "use warnings"
       statements appear earlier in the "program", and that any variables used have already been
       declared, even if those declarations have been omitted to make the example easier to read.

       Do note that the examples have been written by many different authors over a period of
       several decades.  Styles and techniques will therefore differ, although some effort has been
       made to not vary styles too widely in the same sections.  Do not consider one style to be
       better than others - "There's More Than One Way To Do It" is one of Perl's mottos.  After
       all, in your journey as a programmer, you are likely to encounter different styles.

   What is Perl?
       Perl is a general-purpose programming language originally developed for text manipulation and
       now used for a wide range of tasks including system administration, web development, network
       programming, GUI development, and more.

       The language is intended to be practical (easy to use, efficient, complete) rather than
       beautiful (tiny, elegant, minimal).  Its major features are that it's easy to use, supports
       both procedural and object-oriented (OO) programming, has powerful built-in support for text
       processing, and has one of the world's most impressive collections of third-party modules.

       Different definitions of Perl are given in perl, perlfaq1 and no doubt other places.  From
       this we can determine that Perl is different things to different people, but that lots of
       people think it's at least worth writing about.

   Running Perl programs
       To run a Perl program from the Unix command line:

        perl progname.pl

       Alternatively, put this as the first line of your script:

        #!/usr/bin/env perl

       ... and run the script as /path/to/script.pl.  Of course, it'll need to be executable first,
       so "chmod 755 script.pl" (under Unix).

       (This start line assumes you have the env program.  You can also put directly the path to
       your perl executable, like in "#!/usr/bin/perl").

       For more information, including instructions for other platforms such as Windows and Mac OS,
       read perlrun.

   Safety net
       Perl by default is very forgiving.  In order to make it more robust it is recommended to
       start every program with the following lines:

        #!/usr/bin/perl
        use strict;
        use warnings;

       The two additional lines request from perl to catch various common problems in your code.
       They check different things so you need both.  A potential problem caught by "use strict;"
       will cause your code to stop immediately when it is encountered, while "use warnings;" will
       merely give a warning (like the command-line switch -w) and let your code run.  To read more
       about them check their respective manual pages at strict and warnings.

   Basic syntax overview
       A Perl script or program consists of one or more statements.  These statements are simply
       written in the script in a straightforward fashion.  There is no need to have a "main()"
       function or anything of that kind.

       Perl statements end in a semi-colon:

        print "Hello, world";

       Comments start with a hash symbol and run to the end of the line

        # This is a comment

       Whitespace is irrelevant:

        print
            "Hello, world"
            ;

       ... except inside quoted strings:

        # this would print with a linebreak in the middle
        print "Hello
        world";

       Double quotes or single quotes may be used around literal strings:

        print "Hello, world";
        print 'Hello, world';

       However, only double quotes "interpolate" variables and special characters such as newlines
       ("\n"):

        print "Hello, $name\n";     # works fine
        print 'Hello, $name\n';     # prints $name\n literally

       Numbers don't need quotes around them:

        print 42;

       You can use parentheses for functions' arguments or omit them according to your personal
       taste.  They are only required occasionally to clarify issues of precedence.

        print("Hello, world\n");
        print "Hello, world\n";

       More detailed information about Perl syntax can be found in perlsyn.

   Perl variable types
       Perl has three main variable types: scalars, arrays, and hashes.

       Scalars
           A scalar represents a single value:

            my $animal = "camel";
            my $answer = 42;

           Scalar values can be strings, integers or floating point numbers, and Perl will
           automatically convert between them as required.  There is no need to pre-declare your
           variable types, but you have to declare them using the "my" keyword the first time you
           use them.  (This is one of the requirements of "use strict;".)

           Scalar values can be used in various ways:

            print $animal;
            print "The animal is $animal\n";
            print "The square of $answer is ", $answer * $answer, "\n";

           There are a number of "magic" scalars with names that look like punctuation or line
           noise.  These special variables are used for all kinds of purposes, and are documented in
           perlvar.  The only one you need to know about for now is $_ which is the "default
           variable".  It's used as the default argument to a number of functions in Perl, and it's
           set implicitly by certain looping constructs.

            print;          # prints contents of $_ by default

       Arrays
           An array represents a list of values:

            my @animals = ("camel", "llama", "owl");
            my @numbers = (23, 42, 69);
            my @mixed   = ("camel", 42, 1.23);

           Arrays are zero-indexed.  Here's how you get at elements in an array:

            print $animals[0];              # prints "camel"
            print $animals[1];              # prints "llama"

           The special variable $#array tells you the index of the last element of an array:

            print $mixed[$#mixed];       # last element, prints 1.23

           You might be tempted to use "$#array + 1" to tell you how many items there are in an
           array.  Don't bother.  As it happens, using @array where Perl expects to find a scalar
           value ("in scalar context") will give you the number of elements in the array:

            if (@animals < 5) { ... }

           The elements we're getting from the array start with a "$" because we're getting just a
           single value out of the array; you ask for a scalar, you get a scalar.

           To get multiple values from an array:

            @animals[0,1];                 # gives ("camel", "llama");
            @animals[0..2];                # gives ("camel", "llama", "owl");
            @animals[1..$#animals];        # gives all except the first element

           This is called an "array slice".

           You can do various useful things to lists:

            my @sorted    = sort @animals;
            my @backwards = reverse @numbers;

           There are a couple of special arrays too, such as @ARGV (the command line arguments to
           your script) and @_ (the arguments passed to a subroutine).  These are documented in
           perlvar.

       Hashes
           A hash represents a set of key/value pairs:

            my %fruit_color = ("apple", "red", "banana", "yellow");

           You can use whitespace and the "=>" operator to lay them out more nicely:

            my %fruit_color = (
                apple  => "red",
                banana => "yellow",
            );

           To get at hash elements:

            $fruit_color{"apple"};           # gives "red"

           You can get at lists of keys and values with "keys()" and "values()".

            my @fruits = keys %fruit_color;
            my @colors = values %fruit_color;

           Hashes have no particular internal order, though you can sort the keys and loop through
           them.

           Just like special scalars and arrays, there are also special hashes.  The most well known
           of these is %ENV which contains environment variables.  Read all about it (and other
           special variables) in perlvar.

       Scalars, arrays and hashes are documented more fully in perldata.

       More complex data types can be constructed using references, which allow you to build lists
       and hashes within lists and hashes.

       A reference is a scalar value and can refer to any other Perl data type.  So by storing a
       reference as the value of an array or hash element, you can easily create lists and hashes
       within lists and hashes.  The following example shows a 2 level hash of hash structure using
       anonymous hash references.

        my $variables = {
            scalar  =>  {
                         description => "single item",
                         sigil => '$',
                        },
            array   =>  {
                         description => "ordered list of items",
                         sigil => '@',
                        },
            hash    =>  {
                         description => "key/value pairs",
                         sigil => '%',
                        },
        };

        print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";

       Exhaustive information on the topic of references can be found in perlreftut, perllol,
       perlref and perldsc.

   Variable scoping
       Throughout the previous section all the examples have used the syntax:

        my $var = "value";

       The "my" is actually not required; you could just use:

        $var = "value";

       However, the above usage will create global variables throughout your program, which is bad
       programming practice.  "my" creates lexically scoped variables instead.  The variables are
       scoped to the block (i.e. a bunch of statements surrounded by curly-braces) in which they are
       defined.

        my $x = "foo";
        my $some_condition = 1;
        if ($some_condition) {
            my $y = "bar";
            print $x;           # prints "foo"
            print $y;           # prints "bar"
        }
        print $x;               # prints "foo"
        print $y;               # prints nothing; $y has fallen out of scope

       Using "my" in combination with a "use strict;" at the top of your Perl scripts means that the
       interpreter will pick up certain common programming errors.  For instance, in the example
       above, the final "print $y" would cause a compile-time error and prevent you from running the
       program.  Using "strict" is highly recommended.

   Conditional and looping constructs
       Perl has most of the usual conditional and looping constructs.  As of Perl 5.10, it even has
       a case/switch statement (spelled "given"/"when").  See "Switch Statements" in perlsyn for
       more details.

       The conditions can be any Perl expression.  See the list of operators in the next section for
       information on comparison and boolean logic operators, which are commonly used in conditional
       statements.

       if
            if ( condition ) {
                ...
            } elsif ( other condition ) {
                ...
            } else {
                ...
            }

           There's also a negated version of it:

            unless ( condition ) {
                ...
            }

           This is provided as a more readable version of "if (!condition)".

           Note that the braces are required in Perl, even if you've only got one line in the block.
           However, there is a clever way of making your one-line conditional blocks more English
           like:

            # the traditional way
            if ($zippy) {
                print "Yow!";
            }

            # the Perlish post-condition way
            print "Yow!" if $zippy;
            print "We have no bananas" unless $bananas;

       while
            while ( condition ) {
                ...
            }

           There's also a negated version, for the same reason we have "unless":

            until ( condition ) {
                ...
            }

           You can also use "while" in a post-condition:

            print "LA LA LA\n" while 1;          # loops forever

       for Exactly like C:

            for ($i = 0; $i <= $max; $i++) {
                ...
            }

           The C style for loop is rarely needed in Perl since Perl provides the more friendly list
           scanning "foreach" loop.

       foreach
            foreach (@array) {
                print "This element is $_\n";
            }

            print $list[$_] foreach 0 .. $max;

            # you don't have to use the default $_ either...
            foreach my $key (keys %hash) {
                print "The value of $key is $hash{$key}\n";
            }

           The "foreach" keyword is actually a synonym for the "for" keyword.  See ""Foreach Loops"
           in perlsyn".

       For more detail on looping constructs (and some that weren't mentioned in this overview) see
       perlsyn.

   Builtin operators and functions
       Perl comes with a wide selection of builtin functions.  Some of the ones we've already seen
       include "print", "sort" and "reverse".  A list of them is given at the start of perlfunc and
       you can easily read about any given function by using "perldoc -f functionname".

       Perl operators are documented in full in perlop, but here are a few of the most common ones:

       Arithmetic
            +   addition
            -   subtraction
            *   multiplication
            /   division

       Numeric comparison
            ==  equality
            !=  inequality
            <   less than
            >   greater than
            <=  less than or equal
            >=  greater than or equal

       String comparison
            eq  equality
            ne  inequality
            lt  less than
            gt  greater than
            le  less than or equal
            ge  greater than or equal

           (Why do we have separate numeric and string comparisons?  Because we don't have special
           variable types, and Perl needs to know whether to sort numerically (where 99 is less than
           100) or alphabetically (where 100 comes before 99).

       Boolean logic
            &&  and
            ||  or
            !   not

           ("and", "or" and "not" aren't just in the above table as descriptions of the operators.
           They're also supported as operators in their own right.  They're more readable than the
           C-style operators, but have different precedence to "&&" and friends.  Check perlop for
           more detail.)

       Miscellaneous
            =   assignment
            .   string concatenation
            x   string multiplication (repeats strings)
            ..  range operator (creates a list of numbers or strings)

       Many operators can be combined with a "=" as follows:

        $a += 1;        # same as $a = $a + 1
        $a -= 1;        # same as $a = $a - 1
        $a .= "\n";     # same as $a = $a . "\n";

   Files and I/O
       You can open a file for input or output using the "open()" function.  It's documented in
       extravagant detail in perlfunc and perlopentut, but in short:

        open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
        open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
        open(my $log, ">>", "my.log")     or die "Can't open my.log: $!";

       You can read from an open filehandle using the "<>" operator.  In scalar context it reads a
       single line from the filehandle, and in list context it reads the whole file in, assigning
       each line to an element of the list:

        my $line  = <$in>;
        my @lines = <$in>;

       Reading in the whole file at one time is called slurping.  It can be useful but it may be a
       memory hog.  Most text file processing can be done a line at a time with Perl's looping
       constructs.

       The "<>" operator is most often seen in a "while" loop:

        while (<$in>) {     # assigns each line in turn to $_
            print "Just read in this line: $_";
        }

       We've already seen how to print to standard output using "print()".  However, "print()" can
       also take an optional first argument specifying which filehandle to print to:

        print STDERR "This is your final warning.\n";
        print $out $record;
        print $log $logmessage;

       When you're done with your filehandles, you should "close()" them (though to be honest, Perl
       will clean up after you if you forget):

        close $in or die "$in: $!";

   Regular expressions
       Perl's regular expression support is both broad and deep, and is the subject of lengthy
       documentation in perlrequick, perlretut, and elsewhere.  However, in short:

       Simple matching
            if (/foo/)       { ... }  # true if $_ contains "foo"
            if ($a =~ /foo/) { ... }  # true if $a contains "foo"

           The "//" matching operator is documented in perlop.  It operates on $_ by default, or can
           be bound to another variable using the "=~" binding operator (also documented in perlop).

       Simple substitution
            s/foo/bar/;               # replaces foo with bar in $_
            $a =~ s/foo/bar/;         # replaces foo with bar in $a
            $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar
                                      # in $a

           The "s///" substitution operator is documented in perlop.

       More complex regular expressions
           You don't just have to match on fixed strings.  In fact, you can match on just about
           anything you could dream of by using more complex regular expressions.  These are
           documented at great length in perlre, but for the meantime, here's a quick cheat sheet:

            .                   a single character
            \s                  a whitespace character (space, tab, newline,
                                ...)
            \S                  non-whitespace character
            \d                  a digit (0-9)
            \D                  a non-digit
            \w                  a word character (a-z, A-Z, 0-9, _)
            \W                  a non-word character
            [aeiou]             matches a single character in the given set
            [^aeiou]            matches a single character outside the given
                                set
            (foo|bar|baz)       matches any of the alternatives specified

            ^                   start of string
            $                   end of string

           Quantifiers can be used to specify how many of the previous thing you want to match on,
           where "thing" means either a literal character, one of the metacharacters listed above,
           or a group of characters or metacharacters in parentheses.

            *                   zero or more of the previous thing
            +                   one or more of the previous thing
            ?                   zero or one of the previous thing
            {3}                 matches exactly 3 of the previous thing
            {3,6}               matches between 3 and 6 of the previous thing
            {3,}                matches 3 or more of the previous thing

           Some brief examples:

            /^\d+/              string starts with one or more digits
            /^$/                nothing in the string (start and end are
                                adjacent)
            /(\d\s){3}/         three digits, each followed by a whitespace
                                character (eg "3 4 5 ")
            /(a.)+/             matches a string in which every odd-numbered
                                letter is a (eg "abacadaf")

            # This loop reads from STDIN, and prints non-blank lines:
            while (<>) {
                next if /^$/;
                print;
            }

       Parentheses for capturing
           As well as grouping, parentheses serve a second purpose.  They can be used to capture the
           results of parts of the regexp match for later use.  The results end up in $1, $2 and so
           on.

            # a cheap and nasty way to break an email address up into parts

            if ($email =~ /([^@]+)@(.+)/) {
                print "Username is $1\n";
                print "Hostname is $2\n";
            }

       Other regexp features
           Perl regexps also support backreferences, lookaheads, and all kinds of other complex
           details.  Read all about them in perlrequick, perlretut, and perlre.

   Writing subroutines
       Writing subroutines is easy:

        sub logger {
           my $logmessage = shift;
           open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
           print $logfile $logmessage;
        }

       Now we can use the subroutine just as any other built-in function:

        logger("We have a logger subroutine!");

       What's that "shift"?  Well, the arguments to a subroutine are available to us as a special
       array called @_ (see perlvar for more on that).  The default argument to the "shift" function
       just happens to be @_.  So "my $logmessage = shift;" shifts the first item off the list of
       arguments and assigns it to $logmessage.

       We can manipulate @_ in other ways too:

        my ($logmessage, $priority) = @_;       # common
        my $logmessage = $_[0];                 # uncommon, and ugly

       Subroutines can also return values:

        sub square {
            my $num = shift;
            my $result = $num * $num;
            return $result;
        }

       Then use it like:

        $sq = square(8);

       For more information on writing subroutines, see perlsub.

   OO Perl
       OO Perl is relatively simple and is implemented using references which know what sort of
       object they are based on Perl's concept of packages.  However, OO Perl is largely beyond the
       scope of this document.  Read perlootut and perlobj.

       As a beginning Perl programmer, your most common use of OO Perl will be in using third-party
       modules, which are documented below.

   Using Perl modules
       Perl modules provide a range of features to help you avoid reinventing the wheel, and can be
       downloaded from CPAN ( <http://www.cpan.org/> ).  A number of popular modules are included
       with the Perl distribution itself.

       Categories of modules range from text manipulation to network protocols to database
       integration to graphics.  A categorized list of modules is also available from CPAN.

       To learn how to install modules you download from CPAN, read perlmodinstall.

       To learn how to use a particular module, use "perldoc Module::Name".  Typically you will want
       to "use Module::Name", which will then give you access to exported functions or an OO
       interface to the module.

       perlfaq contains questions and answers related to many common tasks, and often provides
       suggestions for good CPAN modules to use.

       perlmod describes Perl modules in general.  perlmodlib lists the modules which came with your
       Perl installation.

       If you feel the urge to write Perl modules, perlnewmod will give you good advice.

AUTHOR
       Kirrily "Skud" Robert <skud AT cpan.org>



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