# warn.pm
package CORE::GLOBAL;

use strict;
use warnings;
use subs 'warn';

sub warn {
    print "WARN '@_'!\n";
    CORE::warn @_;
     }; # die:
    1;

##</code><code>##

# warn.pl
#!/usr/bin/perl

use strict;
use warnings;

warn 'first warn';

use warn;

warn "second warn";

##</code><code>##

first warn at warn.pl line 6.
WARN 'second warn'!
second warn at warn.pm line 9.