Note that the die on $[=$a happens at compile time (run it with -c to see), something you cannot determine of general cases of $$a.

I think *a=*[;$a=5;print $a,$/; is in the same ballpark as perl -wle 'eval q!$[ = 5!; print $[', which also prints 0. After compiling, $[ is read-only, but, as a best effort, Perl ignores an assignment to $[ instead of dying.

Abigail