I'm not sure that I understand why you can't bit-wise xor two strings:

$foo = "baz"; 
$bar = "foo";

print unpack("B*", $foo); 
print unpack("B*", $bar); 
print unpack("B*", ($bar ^ $foo));
__END__
011000100110000101111010
011001100110111101101111
000001000000111000010101
[download]

Seems to do the right thing to me...