Nothing wrong with this. There are way too few factorials that can actually be expressed by Perl...

Perhaps a better question would be to see how well fibonacci or some other function can be sped up, but that too has an answer ...closed form of fibonacci is well known.

Bottom line, for simple math problems, the best answer is always to cheat. This is what calculators do with factorials (and sines and cosines and many other things), and for good reason! Where they don't cheat, they may use simple approximations (taylor's series, etc). But hey, I don't work for Texas Instruments, I just repeat what I am told :)