What Corion said is right.

%temp is actually a container - think of a bucket. You can fill this container (putting water into the bucket), and empty it, which is what %temp = undef does.

Or you can but your bucket into the cupboard, (pushing it to an array), and take a new bucket (declaring %temp inside the loop).

In this real-world analogy you can see that there's a huge difference between these two things.