my %seen_in_1;
$seen_in_1{$_}++ for @rray1;
my @in_2_but_not_1 = grep { not $seen_in_1{$_} } @rray2;
##</code><code>##

my %seen_in_1;
$seen_in_1{$_}++ for @rray1;

my $seen_in_2_but_not_1;
$seen_in_2_but_not_1{$_}++ for grep { not $seen_in_1{$_} } @rray2;
my @in_2_but_not_1 = keys %seen_in_2_but_not_1;