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(1)).
Remember to check the modules that came with your distribution, and
CPAN as well - someone may already have written a
module to do it.