sub difference {
    my $array_ref1 = shift;
    my $array_ref2 = shift;
    my @difference;

    foreach my $element (@$array_ref1, @$array_ref2) { $count{$element}++ };
    foreach my $element (keys %count) {
        push @difference, $element if $count{$element} == 1;
    }
    return \@difference;
}