This ($a,$b) = ($b,$a); is notionally equivalent to

{
    local @list;
    $list[0]=$b;
    $list[1]=$a;
    $a=$list[0];
    $b=$list[1];
    undef @list;
}
[download]

Instead of allocating one temprary scalar and doing 3 assignments, it is allocating a temporary list, performing 4 assignments and deallocating the list.