Simple example of overriding a method in CGI.pm

MyCGI.pm

package MyCGI;
use base 'CGI';

use strict;

our $ShowAll = 0;   # set via $MyCGI::ShowAll

sub a {
    my ( $self, @param ) = @_;

    my $a = $self->SUPER::a(@param);
    my ( $href ) = $a =~ m{href="([^"]*)"};

    $href .= $href =~ /[?]/
        ? "&a=$ShowAll"
        : "?a=$ShowAll";

    $a =~ s{href="[^"]*"}{href="$href"};

    return $a;
}

1;