I don't have an answer either, but following the RT link in that post leads to an issue that was closed in 2013: https://github.com/Perl/perl5/issues/6453. However, I can reproduce the behaviour on Strawberry perl 5.30.

In reply to Re^2: When aliasing sub arguments to @_ elements, PADTMP, READONLY flags are copied inconsistently by swl
in thread When aliasing sub arguments to @_ elements, PADTMP, READONLY flags are copied inconsistently by vr

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