I have found several modules that don't want to build right under a threaded 5.8.0. In my case, I tripped over Crypt::SSLeay and Net::SSLeay. In those two cases, it was a header from the openssl package. I determined that a pre-processor symbol was getting lost under the combination of threading and multi, but I'm too far behind the curve on what threading does/adds to the core to know how to further diagnose it.

--rjray