I had the clever idea of using vec. The Perl code is quite beautiful IMO, but it's slow as a snail. 
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 a
+re 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) },
};

[download]
Results: 
                          Rate               using_vec using_str_bit_o
+ps_and_s
using_vec               51.8/s                      --                
+    -92%
using_str_bit_ops_and_s  672/s                   1196%                
+      --

[download]
12 times slower than ikegami's code I use as a reference implementation.

In reply to Re: Challenge: CPU-optimized byte-wise or-equals (for a meter of beer) by bart
in thread Challenge: CPU-optimized byte-wise or-equals (for a meter of beer) by dragonchild

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