Works perfect. I only changed the testing-lines for better visability:

for (map 1 << $_, 0 .. 63) {
  print sprintf("%b", $_- 1 )," : ", sprintf("%b",nextPowerOfTwo( $_- 
+1 ));
}

$ perl leftmost_bit.pl 
0 : 0
1 : 1
11 : 100
111 : 1000
1111 : 10000
...
[download]

Maybe it's fast enough versus log(),division,log() if inlined. But I am only prototyping the pure Perl version for a C implementation. But it would be nice to have pure Perl as a fallback.