Bit ops? 

sub using_str_bit_ops_and_s {
   my ($s1, $s2) = @_;
   (my $mask = $s1) =~ s/[^\x00]/\xFF/g;
   return ($s1 & $mask) | ($s2 & ~$mask);
}

[download]

tr should be faster. 

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

[download]

Benchmarks: 

            Rate    split1   substr1   bit_ops map_split
split1    1.25/s        --      -95%     -100%     -100%
substr1   25.5/s     1939%        --      -96%      -99%
bit_ops    642/s    51313%     2421%        --      -71%
map_split 2230/s   178496%     8657%      247%        --

[download]

Corion's solution humbles my ass-kicking solution! I wonder if a higher density of zeros would boost my rating?

Update: Added tr variant.
Update: Added Benchmarks.

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

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