#!/usr/bin/perl
# vi: set ts=4 sw=4 sts=2 et:
use strict;
use warnings;


my $text = q~a b c d a b c d~;

$text =~ s/([ab])/&show_what($1)/ge;

print $text, $/;

sub show_what {
    my $match = shift;

    print "matched: $match", $/;
    return $match x 2;
}

__END__