threads::shared can't handle blobs. Whatever they are.

Um, care to elaborate ? Is that "blobs" as in database large binary objects ? Or something else ? A Perl object, perhaps ? Your blanket statement may hide some misassumptions; it certainly makes it difficult to diagnose your issue.

That said, I fear there's currently no unshare API. Though it might be a useful addition, its difficult to define the behavior of something that would be unshared if some other thread still has a reference to it. Does unshare() apply globally, and suddenly every thread's reference becomes private again ? Or does it simply mean the single thread dissociates its private proxy from the shared version, but retains a copy of everything in it ? Plus there may be other housekeeping issues (what happens when you deref a reference to previously shared element; does it grab a copy from the shared version, or the unshared version ?).

If you elaborate on what "blobs" are, it may be possible to find an alternate solution to your dilemma.