How would autovivification know that the undef in lvalue context it's autovivifying wasn't the original undef?

By using a proxy object that acts as undef. Note that you need a reference for autovivification anyway because push @{...} has to modify something that is stored in place of ... anyway.

I don't know if Perl 5 does it that way, but I think kp6 does.

In reply to Re^6: multiple values for one key in hash by moritz
in thread multiple values for one key in hash by sovixi

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