Note that if you disable the shared key hash, then every object will use its own storage for every hash key. For example if a process creates 1000 objects, each being a hash with the same fixed set of keys (but differing values of course), then that process will store 1000 copies of each key.

But on the other hand, using shareable keys on data hashes where sharing is unlikely gives you overhead. If Devel::Size is correct then sharing a key adds 24 bytes.

Maybe it would be nice if it was possible to decide per-hash whether to share keys or not. Or even nicer, if perl objects wouldn't be based on hashes, then key sharing wouldn't be necessary at all.