There are a few applications that perl can be used for, but probably shouldn't. Real-time 3D comes immediately to mind (mostly because I'm a graphics geek, and that's what I do). Sure, I can use the OpenGL or SDL::OpenGL modules, but if I want to push 12 million tris per second I need to worry about vertex formats, video card cache optimization, memory alignment, and so on: sure, I could do that (probably...) in Perl, but it would require far more effort than just doing it in C. Of course, I could write it in C and do XS magic, but then I've done it in C, not Perl....