My list (the list may have items on it for which there are "Perl" modules doing the thing mentioned, but typically such modules contain large parts of C and/or XS code - which isn't Perl).

• Number crunching. Accessing the value of a number requires following at least two pointers, plus some additional overhead to check to see whether the numeric value is correct.
• Anything that requires to be CPU/register/memory/cache friendly. The flexibility of Perl comes with a price, and that's paid with CPU and memory.
• Anything that requires a small footprint in disk space. Compiling C programs with the right compiler can give you programs of just a few k, meaning you can pack a lot of them on a floppy. There's no way to install perl on a floppy.
• Direct memory access, due to the lack of pointers.
• Locking regions of files instead of the entire file.
• Calling kernel hooks.
• Many things for which there are special purpose languages (music, vector images, etc)
• Real time programming
• Threads
• OO

And all other things I forgot.

Abigail