my %seen;
@seen{ @original_1 } = ();
delete @seen{ @original_2 };
my @original_1_only = keys %seen;

##</code><code>##

my %seen;
my @original_1_only;
@seen{ @original_2 } = ();
foreach (@original_1) {
    push(@original_1_only, $_) unless exists $seen{$_};
};