Okay, here's my 2 Cents...

Lately, I've been using TextPad, and although a bit limiting, it works pretty well. Take a look at it for possible functionality suggestions :)

Things to include:

1. Syntax Higlighting (included with textpad)
2. Multiple programing language syntax definitions (included with textpad)
3. Regexp Searches (included with textpad)
4. UNIX / Windows compatibility (please please please!!!)
5. The ability to run command line tools (sorta included with textpad)

I like your suggestion of interpreting perl on the fly... that'd be waaay coool... if you do it modularly, you could somehow plug in other interpreters, such as javascript, or Basic, or even java (you know that the JVM is just an interpreter, right?)

My apologies for this sounding like an ad for textpad... I was using it to compare functionality, and am not trying to advocate it. I would use Emacs or Xemacs, but it looks like those were already covered :)

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E-Bitch
Tempora Mutantur Nos et Mutamur in Illis
"The Times are Changed Even as We are Changed in Them"