Bisecting shows this bug was fixed by 4596056478d3a as a fix for RT#31865, and the fix was part of the Perl v5.14.0 release:

When assigning a list with duplicated keys to a hash, the assignment used to return garbage and/or freed values:

@a = %h = (list with some duplicate keys);

This has now been fixed.

In reply to Re: Missing Hash Keys When Hash Created in Sub by Anonymous Monk
in thread Missing Hash Keys When Hash Created in Sub by Stringer

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