Overriding Built-in Functions - Page 5

Another way to predeclare subroutines is with the use subs pragma. This not only predeclares the subroutine, but also allows us to override Perl's existing built-in functions and replace them with our own. We can access the original built-in function with the CORE:: prefix. For example, here is a replacement version of the srand function, which issues a warning if we use srand in a version of Perl of 5.004 or greater without arguments (see Appendix C for more on the srand function):

#!/usr/bin/perl
# srandcall.pl
use warnings;
use strict;
use subs qw(srand);

sub srand {
  if ($] >= 5.004 and not @_) {
    warn "Unqualified call to srand redundant in Perl $]";
  } else {
   # call the real srand via the CORE package
    CORE::srand @_;
  }
}

Now if we use srand without an argument and the version of Perl is 5.004 or greater, we get a warning. If we supply an argument we are assumed to know what we are doing and are supplying a suitably random value.

Subroutines like this are generally useful in more than one program, so we might want to put this definition into a separate module and use it whenever we want to override the default srand:

#!/usr/bin/perl
# mysrand.pm

package mysrand;

use strict;

use vars qw(@ISA @EXPORT @EXPORT_OK);
use Exporter;

@ISA = qw(Exporter);
@EXPORT = qw(mysrand);
@EXPORT_OK = qw(srand);

sub mysrand {
  if ($] >= 5.004 and not @_) {
    warn "Unqualified call to srand redundant in Perl $]";

  } else {
  # call the real srand via the CORE package
  CORE::srand @_;
  }
}

use subs qw(srand);
sub srand {&mysrand;};
# pass @_  directly to mysrand

This module, which we would keep in a file called mysrand.pm to match the package name, exports the function mysrand automatically, and the overriding srand function only if we ask for it.

use mysrand; # import 'mysrand'
use mysrand qw(mysrand); # import and predeclare mysrand;
use mysrand qw(srand); # override 'srand'

We'll talk about packages, modules, and exporting subroutines in Chapter 10.

Strict Subroutines and the 'use strict subs' Pragma - Page 4
Professional Perl Programming
The Subroutine Stack - Page 6