Let's see if I can come up with something you don't already know this time...

The issue with support on Cache::FastMmap is likely that it uses unsupported-on-Windows C-code to handle file locking/unlocking. In the "Modules like this" section of the documentation, it references Cache::Mmap, which is a pure Perl solution, and thus should respect (to the extent possible) Win32 behaviors. A quick glance at the source suggests it should be fine with threading.