xdg provided the original insight on how to make inside-out objects thread-safe in Threads and fork and CLONE, oh my!.

Though it bears repeating that this is only possible on Perl 5.8 or better which supports the special CLONE method.

Whether using threads or threads::shared in Perl is a good idea or not is a whole separate issue. (c.f. Things you need to know before programming Perl ithreads and Re: Reliable asynchronous processing). However, it's important for people to realize that forking on Win32 actually uses threads (c.f. perlfork) so this behavior of inside-out objects needs to be continually flagged.

None of these are "deal-killers" for inside-out objects, but they do require awareness and management of complexity. That said, I think jdhedden has done a great job with Object::InsideOut (as he did with Math::Random::MT::Auto) in creating something that might well serve as the gold-standard in the category.