For a very large proportion of uses of fork -- essentially all those using the 'fork & exec' idiom -- system, especially system 1, ... is a far more effective 'emulation'. The spawned process can (by default does) inherit many of the system resources -- open file, pipe and socket handles and the like -- that forked processes inherit under *nix.

But, as Corion correctly points out, there are limitations that prevent it from being a transparent solution to porting forking programs to Windows.

It is possible to provide a more realistic and efficient fork emulation on Windows -- one that uses real processes and even COW memory. Creating the new process and giving it COW access to the parents memory segments is relatively trivial. The difficult part is fixing up the perl internals within the spawned copy.