Thanks sauoq.

That's a minimal demonstration of the problem.

Yep. I guess we arrived to the problem quite close. IO::Zlib does not seem to implement fileno. But even after faking it (as shown in my Updated node), there is something else going bad.

(So, you can stop looking at Storable as the real problem, anyway.)

Well, I don't understand why Storable wants to verify that it got a real handle. It should try to do its thing with whatever it got as an argument.

But in this case, this does not look like the cause.