use strict;
use warnings;

my @db = (1, 3, 5, 7, 9, 11);
my @in = (1, 2, 5, 8, 9, 10, 13);
my %lookup;
my @result;

@lookup{@in} = ();
foreach my $elem (@db) {
  push(@result, $elem) unless exists $lookup{$elem};
}

print "@result\n"; # should print 3 7 11.