Well, there are certainly times when you don't want to use Perl. I don't see the Linux kernel being rewritten any time soon. If you need something that's as fast as it can possibly be, or want lower level control over the internels of the computer, something like C would probably be a better choice. Even Perl itself is written in C (and Parrot too).

That said, most everything I've developed has worked just fine under Perl and was probably quite a bit easier to write than if I'd used another language.