use feature qw( state );
   use Type::Params qw( signature );
   use Types::Standard qw( Num );
   
   sub add_numbers {
      state $signature = signature(
         method     => 1,
         named      => [ 'x' => Num, 'y' => Num ],
      );
      my ( $self, $arg ) = $signature->( @_ );
      
      return $arg->x + $arg->y;
   }