sub using_vec {
   my ($s1, $s2) = @_;
   vec($s1, $_, 8) ||= vec($s2, $_, 8) for 0 .. length($s1)-1;
   return $s1;
}


sub using_str_bit_ops_and_s {
   my ($s1, $s2) = @_;
   (my $mask = $s1) =~ tr/\x00/\xFF/c;
   return ($s1 & $mask) | ($s2 & ~$mask);
}

my $x = pack 'C*', map int rand(256), 1 .. 64*1024;
my $y = pack 'C*', map 1+int rand(255), 1 .. 64*1024;

using_str_bit_ops_and_s($x, $y) eq using_vec($x, $y) or die "Results are different??";

use Benchmark 'cmpthese';
cmpthese -3, {
  using_str_bit_ops_and_s => sub{ my $r = using_str_bit_ops_and_s($x, $y) },
  using_vec=> sub{ my $r = using_vec($x, $y) },
};