$msg = "this is a test messagez";
$key = "\x4f\xcb";   # or  $key = pack "n", 0x4fcb;

$xored = $msg ^ $key;

print join("  ", split(//, $msg)), "\n";
print join(" ", unpack("(H2)*", $msg)),   "\n";
print join(" ", unpack("(H2)*", $key)),   "\n";
print join(" ", unpack("(H2)*", $xored)), "\n";

__END__
t  h  i  s     i  s     a     t  e  s  t     m  e  s  s  a  g  e  z
74 68 69 73 20 69 73 20 61 20 74 65 73 74 20 6d 65 73 73 61 67 65 7a
4f cb
3b a3 69 73 20 69 73 20 61 20 74 65 73 74 20 6d 65 73 73 61 67 65 7a

##</code><code>##

    $key = "\x4f\xcb" x (length($msg)/2);
or
    $key = pack("n", 0x4fcb) x (length($msg)/2);

##</code><code>##

t  h  i  s     i  s     a     t  e  s  t     m  e  s  s  a  g  e  z
74 68 69 73 20 69 73 20 61 20 74 65 73 74 20 6d 65 73 73 61 67 65 7a   $msg
4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb      $key
3b a3 26 b8 6f a2 3c eb 2e eb 3b ae 3c bf 6f a6 2a b8 3c aa 28 ae 7a   XOR result