Should cvcache be per CV (keyed by thread) or per thread (keyed by CV)?

Maybe ought to protect all SVs by a mutex for SvREFCNT_{dec,inc},
upgrades and so on. Then use SvMUTEX instead of CvMUTEX for CVs.
On the other hand, people shouldn't expect concurrent operations
on non-lexicals to be safe anyway.

Probably don't need to bother keeping track of CvOWNER on clones.

Either @_ needs to be made lexical or other arrangments need to be
made so that some globs (or just *_) are per-thread.

tokenbuf and buf probably ought to be global protected by a global lock.
