Perl isn't great at numerical analysis. For example, if you have a 1024x1024 matrix (possibly for a fourier analysis) it will eat up far more memory in perl than it will in C/C++. C/C++ isn't necessarily the best choice for this example either. There are modules that make it easier and more memory efficient, but I still don't think perl will the best tool for this.
Low level image processing in perl isn't that great either. There are a lot of modules in CPAN that interface to external utilities (like Image::Magick).
I can't really imagine anyone wanting to write a device driver in perl :)
All of this is IMHO of course. There might be really good ways to do all of these things with modules.