I am fairly sure that that is an optimization introduced in 5.8 to reduce memory usage with the common case of hashes used to implement objects. I believe that this optimization was part of the strategy for replacing the pseudo-hashes that were experimental in 5.6 and removed (if I recall correctly) in 5.10.

Pseudo-hashes were an attempt at reducing the overhead for structures with named fields by allowing multiple arrays to share a hashref in position zero which mapped keys to array indices.

In reply to Re: do separate hashes share common keys? by jcb
in thread do separate hashes share common keys? by LanX

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