It doesn't answer my question... I want to know what return () does... For example, if I'm correct, return [] returns an array ref right? So, does return () return a subroutine ref?

Yes,  [ ] is an array ref, and no () is not a reference of any kind (not a subroutine ref)

References are explained in perlreftut#Making References , references quick reference

These are all the same (as documente in return, they return nothing (empty list) or undef , depending on context (list or scalar) )

sub roy { return;   }
sub roy { return    }
sub roy { return(); }
sub roy { return()  }
sub roy { return ()  }
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These are all the same (return an array ref)

sub roy { return [];  }
sub roy { return []   }
sub roy { return([]); }
sub roy { return([])  }
sub roy { return  ([])  }
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See Tutorials: Context in Perl: Context tutorial, "List" Is a Four-Letter Word, Easy Reference for Contexts