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

@lookup{@in} = ();

@lookup1{@db} = ();

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

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