Current Perl documentation can be found at perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
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
the perlfunc manpage).
Remember to check the modules that came with your distribution, and CPAN as well - someone may already have written a module to do it.