Yes , did you?

Yes.

There's nothing in the linked post to support your false that Perl must execute the shift(@_) before the @_ on the right.

Nowhere in perltoot or in the other document you linked is @_ both used and modified in the same statement.

None of the three links show what you claimed they do.

And still, its used all over CORE

Thank you. This is all I asked for. So much code using that undocumented behaviour means it's not likely to change, at least not without a good long deprecation cycle.