> I still do not understand what is going on when python executed thic 
> code. I have a local variable l in my parent process. 

No, you don't. It's a global variable, not a local one.

> When I create a 
> child process, program makes first makes a copy of memory. Than what?

It doesn't have to do anything with the multiprocessing at all.
For comparison, just run the Python script

def f():
  del l
l = []
f()

It produces the same error, with no multiprocessing involved.