$RESPONSE is a reference and it is basically a memory address. Now it's great to pass that around instead of copying the actual value which is one of the reasons people use it. Now, you can't just say something like $RESPONSE = 0 because that would assign the memory address of $RESPONSE to 0... not the actual value. So in order to dereference it (or in other words to use the actual value) you put a $ in the front. This basically says, I don't want the address, instead I want the value at that address. In your case you are assigning the value of 0 to it instead of getting a value. It works both ways.

Hope that helped. The docs are pretty good too, I just thought I would give a brief description.