And I don't see how you could really do justice to such a tool without implementing it in Perl, or at least delegating key portions to an embedded Perl instance.

For most of the things you list, a Perl implementation isn't required--you just need the ability to parse Perl. In fact, since some of them will probably require hooks into the perl interpreter (or maybe Parrot), it's preferable to do much of it in C. In particular, the run-time debugger and symbol table lookups will probably be best with interpreter hooks.

Test script support is probably best done by hooking into Test::Harness, unless there is a particularly good reason to come up with something else. It'd also be nice to be able to write plugins in Perl. However, neither of these things make it absolutely required to write entire editor in Perl (as long as proper hooks are in place).

I'm not saying a editor written in Perl is a bad idea, just that I don't see what benefit it will bring to the user over any other langauge. There are already too many text editors/IDEs out there, and the same people who keep using vi or emacs will probably keep using them.