my $m =
    $z > 0x00000000ffffffff
      ? $z > 0x0000ffffffffffff
        ? $z > 0x00ffffffffffffff
          ? $z > 0x0fffffffffffffff
            ? $z > 0x3fffffffffffffff
              ? $z > 0x7fffffffffffffff ? 0x8000000000000000 : 0x4000000000000000
              : $z > 0x1fffffffffffffff ? 0x2000000000000000 : 0x1000000000000000
... etc.