phpman > man > perlpragma(1)

PERLPRAGMA(1)                     Perl Programmers Reference Guide                     PERLPRAGMA(1)



NAME
       perlpragma - how to write a user pragma

DESCRIPTION
       A pragma is a module which influences some aspect of the compile time or run time behaviour
       of Perl, such as "strict" or "warnings". With Perl 5.10 you are no longer limited to the
       built in pragmata; you can now create user pragmata that modify the behaviour of user
       functions within a lexical scope.

A basic example
       For example, say you need to create a class implementing overloaded mathematical operators,
       and would like to provide your own pragma that functions much like "use integer;" You'd like
       this code

           use MyMaths;

           my $l = MyMaths->new(1.2);
           my $r = MyMaths->new(3.4);

           print "A: ", $l + $r, "\n";

           use myint;
           print "B: ", $l + $r, "\n";

           {
               no myint;
               print "C: ", $l + $r, "\n";
           }

           print "D: ", $l + $r, "\n";

           no myint;
           print "E: ", $l + $r, "\n";

       to give the output

           A: 4.6
           B: 4
           C: 4.6
           D: 4
           E: 4.6

       i.e., where "use myint;" is in effect, addition operations are forced to integer, whereas by
       default they are not, with the default behaviour being restored via "no myint;"

       The minimal implementation of the package "MyMaths" would be something like this:

           package MyMaths;
           use warnings;
           use strict;
           use myint();
           use overload '+' => sub {
               my ($l, $r) = @_;
               # Pass 1 to check up one call level from here
               if (myint::in_effect(1)) {
                   int($$l) + int($$r);
               } else {
                   $$l + $$r;
               }
           };

           sub new {
               my ($class, $value) = @_;
               bless \$value, $class;
           }

           1;

       Note how we load the user pragma "myint" with an empty list "()" to prevent its "import"
       being called.

       The interaction with the Perl compilation happens inside package "myint":

           package myint;

           use strict;
           use warnings;

           sub import {
               $^H{"myint/in_effect"} = 1;
           }

           sub unimport {
               $^H{"myint/in_effect"} = 0;
           }

           sub in_effect {
               my $level = shift // 0;
               my $hinthash = (caller($level))[10];
               return $hinthash->{"myint/in_effect"};
           }

           1;

       As pragmata are implemented as modules, like any other module, "use myint;" becomes

           BEGIN {
               require myint;
               myint->import();
           }

       and "no myint;" is

           BEGIN {
               require myint;
               myint->unimport();
           }

       Hence the "import" and "unimport" routines are called at compile time for the user's code.

       User pragmata store their state by writing to the magical hash "%^H", hence these two
       routines manipulate it. The state information in "%^H" is stored in the optree, and can be
       retrieved read-only at runtime with "caller()", at index 10 of the list of returned results.
       In the example pragma, retrieval is encapsulated into the routine "in_effect()", which takes
       as parameter the number of call frames to go up to find the value of the pragma in the user's
       script. This uses "caller()" to determine the value of $^H{"myint/in_effect"} when each line
       of the user's script was called, and therefore provide the correct semantics in the
       subroutine implementing the overloaded addition.

Key naming
       There is only a single "%^H", but arbitrarily many modules that want to use its scoping
       semantics.  To avoid stepping on each other's toes, they need to be sure to use different
       keys in the hash.  It is therefore conventional for a module to use only keys that begin with
       the module's name (the name of its main package) and a "/" character.  After this module-
       identifying prefix, the rest of the key is entirely up to the module: it may include any
       characters whatsoever.  For example, a module "Foo::Bar" should use keys such as
       "Foo::Bar/baz" and "Foo::Bar/$%/_!".  Modules following this convention all play nicely with
       each other.

       The Perl core uses a handful of keys in "%^H" which do not follow this convention, because
       they predate it.  Keys that follow the convention won't conflict with the core's historical
       keys.

Implementation details
       The optree is shared between threads.  This means there is a possibility that the optree will
       outlive the particular thread (and therefore the interpreter instance) that created it, so
       true Perl scalars cannot be stored in the optree.  Instead a compact form is used, which can
       only store values that are integers (signed and unsigned), strings or "undef" - references
       and floating point values are stringified.  If you need to store multiple values or complex
       structures, you should serialise them, for example with "pack".  The deletion of a hash key
       from "%^H" is recorded, and as ever can be distinguished from the existence of a key with
       value "undef" with "exists".

       Don't attempt to store references to data structures as integers which are retrieved via
       "caller" and converted back, as this will not be threadsafe.  Accesses would be to the
       structure without locking (which is not safe for Perl's scalars), and either the structure
       has to leak, or it has to be freed when its creating thread terminates, which may be before
       the optree referencing it is deleted, if other threads outlive it.



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