When shouldn't I program in Perl?

Actually, one good reason is when you already have an existing application written in another language that's all done (and done well), or you have an application language specifically designed for a certain task (e.g. prolog, make).

For various reasons, Perl is probably not well-suited for real-time embedded systems, low-level operating systems development work like device drivers or context-switching code, complex multithreaded shared-memory applications, or extremely large applications. You'll notice that perl is not itself written in Perl.

The new native-code compiler for Perl may reduce the limitations given in the previous statement to some degree, but understand that Perl remains fundamentally a dynamically typed language, and not a statically typed one. You certainly won't be chastized if you don't trust nuclear-plant or brain-surgery monitoring code to it. And Larry will sleep easier, too -- Wall Street programs not withstanding. :-)