Good point.

However even so I would argue against turning it on naively. You see in Perl part of cleaning up a call-frame is destroying temporary lexical variables and undoing invocations of local. When the things destroyed are objects that may have side-effects in their DESTROY methods (see ReleaseAction for an example where this happens) then trying to optimize tail-recursion creates a significant change in what the code does.

Which brings us to another theme. A lot of the theory folks argue for side-effect free programming (this is what "functional programming" really means - as opposed to the bad meme that I started) because (among other things) it gives the compiler far greater opportunity to optimize and reorder how things happen. (Proponents argue that it also makes big programs easier to understand.) However we naive humans find avoiding side-effects a difficult discipline, which makes a lot of theoretical optimizations very hard to apply in practical languages.

As Larry Wall said, optimization is a form of cheating, which is why you always get caught. And the more free-form the language is, the more requirements you put on the compiler, and the easier it is to catch it out when you try to cheat. :-)