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package Quantity;

...
  $self->new($self->{quantity + $target->{quantity},
             $self->{unit});
}

  $a = Quantity->new(10, 'apple');
  $b = Quantity->new(20, 'orange');
...
  ...

  $c = $a + $b; # dies "Oi! you can't add apples to oranges!"