upon investigation, it turns out that when inside map, you can not refer to the array (or hash) you are assigning to at all

Of course you can't. It doesn't exist yet.

$a = $a + 7;

This code evaluates the RHS first, then assigns to $a. In the warped logic you propose, $a = ($a = $a + 7) + 7 ad infinitum :)

You can of course refer to the array in an assignment when values already exist for the parts of the array you're referring to.