It's further worth noting that DigitalKitty's algorithm uses only simple tail recursion, which can be easily compiled to be as fast or faster than an iterative solution.

Although I'm no expert, I believe the compiler would actually have to be smarter to optimize your iterative solution compared to the tail recursive one. Making the inference that $lower and $higher are candidates for register variables is slightly easier to make in the tail recursive version as there is a tendency to use registers for the subroutine calls variables for low-arity functions.

I could be wrong there, but a good compiler could do about as well with either, I would bet.

Now, implementing full tail call elimination is trickier, but recognizing tail recursion in this case and performing the optimization is a fairly trivial thing to do. Of course, we are talking about perl code here, and perl does not perform this optimization...