#!/usr/bin/perl -w
use strict;

my $text = do { local $/; <DATA> };

$text =~ s/my.*?\((\w+)\)/print "$1\n"; $1/ge;

__DATA__
my dog is (red1)
my cat is (blue1)
my dog is (red2)
my cat is (blue2)
my dog is (red3)
my cat is (blue3)
my dog is (red4)
my cat is (blue4)
my dog is (red5)
my cat is (blue5)
my dog is (red6)
my cat is (blue6)