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

my @list1 = (1, 2, 3, 4, 5);
my @list2 = (2, 3, 4);

my @diff;
my %repeats;

for (@list1, @list2) { $repeats{$_}++ }
for (keys %repeats) {
	push @diff, $_ unless $repeats{$_} > 1;
}