x   y
a b 1 2 3 4 5 6 7

    x   y
1 2 a b 3 4 5 6 7  -- recurse

        x   y
1 2 3 4 a b 5 6 7  -- recurse
            x   y
1 2 3 4 5 6 a b 7
              x   y
1 2 3 4 5 6 7 b a  -- first swap on this recusion level
              x y
1 2 3 4 5 6 7 b a  -- restore y's position
              x   y
1 2 3 4 5 6 7 a b  -- second swap and recurse

$y == @$ary now, so stop.