I don't think Perl's speed (as in execution time) is a major concern for most AI programming. It's frequently noted that developer time is often far more valuable then execution time for other projects, why would AI programming be any different?
Inertia, on the other hand, would definately play a part. If you've been programming in Lisp your entire life, why not continue to use it? Does Perl really offer enough advantages in this area for it to be worth learning? Even if it did, chances are the learning curve would reduce your productivity for an extended period of time (you mean every second character doesn't have to be a parenthesis ?).
That said, not everyone starts programming with Lisp or Java, in fact many start with Perl. Is there any reason why they shouldn't continue to use Perl? Why would we not expect to see its uses in AI gradually increase?