Thanks for your answers.
I already changed my script to moritz suggestion. This is a much better approach and allows a more exact control for timing.

A script that can handle threads is the way it should go, but for this I must rewrite some other parts of the script regarding killing the running processes etc...

I do not know where I read about the memory and cpu consumption of sleep. Actually I googled around and could not verify my statement. So probably this is not true (Although I remember one of my scripts using quite a lot cpu or mem while sleeping, but this can also have other reasons).Anyway the alternatives are great and improved my script very much.

I searched CPAN before writing my script and could not find anything suitable. Schedule::At seems to do what I need. I will give it a second thought.