man pages section 1: User Commands

Exit Print View

Updated: July 2014

perlpragma (1)


perlpragma - how to write a user pragma


Please see following description for synopsis


Perl Programmers Reference Guide                    PERLPRAGMA(1)

     perlpragma - how to write a user pragma

     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

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:

perl v5.12.5         Last change: 2012-11-03                    1

Perl Programmers Reference Guide                    PERLPRAGMA(1)

         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;


     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} = 1;

         sub unimport {
             $^H{myint} = 0;

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


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

perl v5.12.5         Last change: 2012-11-03                    2

Perl Programmers Reference Guide                    PERLPRAGMA(1)

         BEGIN {
             require myint;

     and "no myint;" is

         BEGIN {
             require myint;

     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} when
     each line of the user's script was called, and therefore
     provide the correct semantics in the subroutine implementing
     the overloaded addition.

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 v5.12.5         Last change: 2012-11-03                    3

Perl Programmers Reference Guide                    PERLPRAGMA(1)

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

perl v5.12.5         Last change: 2012-11-03                    4