In fact, it depends on the module in question. But, if the module uses Exporter, the second option only imports is_valid into the current namespace. All other functions still exist under their fully qualified names, so the memory should not matter. But, if the module exports a subroutine called "all" or something similarly simple, you can avoid a clash with your own "all" subroutine. The qw way keeps your namespace clean and under better control.
in reply to Difference between use Module::Name and use Module::Name qw /method1 method2/