In most cases, you write an external module to do it--see the answer to "Where can I learn about linking C with Perl? [h2xs, xsubpp]". However, if the function is a system call, and your system supports syscall(), you can use the syscall function (documented in perlfunc).
Remember to check the modules that came with your distribution, and CPAN as well--someone may already have written a module to do it.