though it looks like puke, it's a single statement. Is there a better performing solution?

I doubt there will be a massively better performing solution but unless you are doing this literally billions of times I'd caution against this sort of code because it is such a maintenance nightmare.

Here's a nice, clean bitwise shift version. Likely much slower but so much more maintainable.

my $z = 256; # input
my $m = 0;
$m++ while $z >> $m;
say $m;
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Update: See also Bit::Util::bu_first by salva.