You probably want something like this: use a hash to record all the items in the first array, then grep for items in the second array that you haven't already seen:

my %seen_in_1;
$seen_in_1{$_}++ for @rray1;
my @in_2_but_not_1 = grep { not $seen_in_1{$_} } @rray2;
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Or, to avoid duplicates:

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;
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