%hash = () is slower than undef %hash about 1.2 times [...] What is your opinion?

It seems you are micro-optimizing. Unless %hash=() resp. undef %hash takes significant amounts of time (say a minute or more per run), I would simply ignore it. Very likely, other places of your code need much more time and optimizing them gains much more. Hint: Try Devel::NYTProf.

Alexander