The third line of arrays ends in a period (the contatenation operator) rather than a comma (list argument separator). From my limited knowledge of the internals, I would guess this would cause the third and fourth arrays to be placed in string context and concatenated (returning something like ARRAY($mem_add1)ARRAY($mem_add2)). Thus, your 5 element array would contain only 4 elements: 2 AoAs, a meaningless string, and one more AoA.