Perl's threading model is unstable, differs with every major release, and Perl 6 (which is projected to be stable) will undoubtably have yet another model. In short you don't really want to use it.

Beyond that, the clone() call is very, very much Linux specific. Nobody else does things that way, and if you wish to write portable code in C you won't either.

Besides which, threaded code is definitely a mixed blessing. There are definitely times when multi-threading is the right thing to do. But I get very wary when I see people pulling it out as an answer everywhere. See Threads vs Forking (Java vs Perl) for further discussion on why.

UPDATE
Linking by ID seems to be broken at the moment. The missing thing to try for further discussion is Threads vs Forking (Java vs Perl)...