Although I normally neither use nor need prototypes in Perl, I'd still love to see them being extended in such a way that you could make regular Perl functions behave like Perl builtins in every respect (think of overriding print, system, etc.).  I've come across some discussion along these lines on p5p (e.g. here), but I think things never went beyond just being discussed.

(This is not my entire Perl 5 wishlist, btw, but the other items are still too vague / not thought-out enough yet, that I would dare to present them to this audience :)