This doesn't work, since $self = Tie::RefHash::Nestable->TIEHASH($self, $class); returns a Tie::RefHash::Nestable-object - but what I want to get back is a GlobalHash object ...

By reblessing the $self object into the GlobalHash class you would be able to return its object as shown below, wouldn't you?

bless $self, 'GlobalHash';
return $self;