It only occured to me today that you're wrong. it is impossile for the output to be ab.

If the left one is evaluated first, the first element in the pair is a. The second element if also a, but $i gets incremented to 1. In this case the result is 'aa'.

Alternately, if the right one is evaluated first, the second element in the pair is a, and $i gets incremented to 1. The first element is then set to b,for a final result of 'ba'.

As far as I can see, there would be no way to wind up with 'ab'