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

##</code><code>##

@lookup{@in} = ();

##</code><code>##

@lookup1{@db} = ();

##</code><code>##

my %lookup;
@lookup{@in} = ();
foreach my $elem (@db) {
  if (exists $lookup{$elem}) { 
  @db = grep {not exists $lookup{$_}} @db;
}
}

##</code><code>##

@lookup{@db} = ();
foreach my $elem (@in) {
  if (not exists $lookup{$elem}) { 
  push(@db, $elem);
}
}