Some people have already convinced themselves that they know exactly what order things are done and why and even say things like "It must be this way to allow ..."

I don't know how I could have convinced myself I "know exactly what order things are done and why" when I specifically mentioned I didn't know whether execution order is defined in this circumstance.

"It" in "It must be this way to allow ..." refers to passing by reference, not execution order. There is no way of allowing $_[0] = ... to edit the caller's variable without passing the variable in by reference (by definition). Perl5 doesn't currently have a mechanism to specify whether a variable should be passed by value or by reference (or maybe it does and it wasn't used here), so everything must be passed by reference to allow $_[0] = ... to edit the caller's variable.

My post only explains why ++$i affected the $i, nothing more.