What you may not know about the return function is that it evaluates its parameter(s) in the context of the subroutine invocation.

If you called the subroutine like this: $f= sub(...) then %filters will be evaluated in a scalar context and return a string that shows some statistics of the hash, but no values of the hash itself.

To rectify this you can do one of two things: Return a reference to the hash instead of the hash, i.e. "return \%filters;". Or change the call context, i.e. "%f= sub(...)". The second method copies the complete hash and should be avoided if the hash is really big