#!/usr/bin/env perl -l

use strict;
use warnings;

my %seen;

for (qw{john.co.uk gim.com}) {
    ++$seen{get_first_part_of_address($_)};
}

for (qw{john.com gim elephant.com banana}) {
    print unless $seen{get_first_part_of_address($_)};
}

sub get_first_part_of_address {
    return (split /\./, shift)[0]
}

##</code><code>##

elephant.com
banana