You can do almost anything in Perl but some things work better in C. Operating systems and device drivers are two examples. Encryption algorithms are an example where the Perl C XS interface is useful. The main code is C with a Perl wrapper. While you can implement almost any algorithm in Perl, it often makes more sense to wrap well tested, and freely available, C library code. That way you get easy access from Perl and don't have to reininvent the wheel.