in reply to What Perl CAN'T do?
It's not so much what Perl can't do, as what can be done better other ways. Other replies have pointed out that Perl doesn't play so well close to the metal.
Actually Perl doesn't play so well at the other end of the programming spectrum either - 1,000,000 line applications would be somewhat difficult to manage in Perl. And there are real applications that are 1,000,000 lines long because they have to be, not because they are so poorly written that in a realTM language like Perl they would be only 10,000 lines, but because that is a fair representation of their required complexity. Even 10,000 lines of Perl would take a bit of taming. :)
The answer to your updated question is: almost anything dealing directly with the computer hardware without assistance from the operating system. Things like kernel code and device drivers. Bear in mind too that a lot of the "good stuff" in Perl is provided by modules and much of the really fast good stuff is XS code - that is written in C, so isn't really Perl at all in some sense.
DWIM is Perl's answer to Gödel