Do you mean it increase by 4 bytes once, or every single time it goes through the loop?

I don't follow your 'settling in' argument.

Perl need a certain amount of memory to do what you asked for. Taking that amount is not a "leak" but just the normal process of running a program. After allocating enough memory, the program should "settle in" and not need to get more unless your data grows.

On the other hand, if the program grows constantly when you aren't adding any data, that would be a leak. If it really leaks, you should be able to run that loop tye posted until it consumes all the memory on your machine and goes into swap.