Are you on Windows? Forking is mostly faked on Windows. Are you loading any modules before using threads? Threads cause all loaded code to be copied into each spawned interpreter. Did you try a simple $pid = fork() approach? There may be overhead in forks.pm due to the emulation of the threads API.
Comment on Re: Re: Re: latest on ithreads vs forks?
The latest versions of forks do not install on Win32 at all anymore. It just doesn't make sense. It also didn't work. And I couldn't be bothered to find out why, as it doesn't make any sense to use forks.pm on Win32 systems (with fork() being emulated in threads).