in reply to Re^3: Unexpected parsing
in thread Unexpected parsing

That one's simply undocumented.
From Auto increment and Auto decrement:
Note that just as in C, Perl doesn't define when the variable is incremented or decremented. You just know it will be done sometime before or after the value is returned. This also means that modifying a variable twice in the same statement will lead to undefined behaviour.
(with the specific example I mentioned listed under “Don't do that”).