use experimental qw( signatures );
   use Type::Params qw( signature_for );
   use Types::Standard qw( Num );
   
   signature_for add_numbers => (
      method     => 1,
      named      => [ 'x' => Num, 'y' => Num ],
   );
   
   sub add_numbers ( $self, $arg ) {
      return $arg->x + $arg->y;
   }