t's good news that fork seems to only clone the current thread, but, like you say, it sounds very system-dependant, and inconsistent.

Well, your module definitely wouldn't work on Win32, and given the problems some other, non-win32 people have using the Expect module, it seems unlikely that your delightfully simple module will be sufficiently robust for widespread portability.

Given that

• (as you said in your OP), system in a threads appears to work on your system;
• that the docs for POSIX/Solaris indicate that it should work on those systems;
• and it does work on Win32;
• Up-to-date builds of Linux (post 2.6 kernel?) offer NTPL (Native POSIX Threads Libraries) with much closer POSIX conformance than the older LinuxThreads;

it seems to me that rather than writing a potentially complex module (see Expect.pm) for this, the right solution would be to verify that it does work correctly on POSIX/Solaris systems and remove the dire warning you quoted in your OP, from perlthrtut.

It might need to be replaced with a list of systems on which it definitely doesn't work correctly, or maybe just a few limitations on specific systems.