In terms of documentation, the perldelta for 5.20.0 notes that "for example hash keys were already copy-on-write" when it describes the COW implementation.

https://metacpan.org/pod/distribution/perl/pod/perl5200delta.pod#Internal-Changes

I'm not sure when it was introduced, but jcb's estimate in 11128352 looks reasonable to me.

(Edit: look for the ninth bullet point under the link to find the quoted text).

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

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