in reply to OO, Class::MethodMaker, and get_set

I'd do it something like this in Lisp (not very tested):

(defclass sum () ((a :accessor a :initarg :a :initform 0) (b :accessor b :initarg :b :initform 0) (c :reader c))) (defmethod compute ((self sum)) (setf (slot-value self 'c) (+ (a self) (b self)))) (defmethod initialize-instance :after ((self sum) &key &allow-other-ke +ys) (compute self)) (defmethod (setf a) :after (val (self sum)) (declare (ignore val)) (compute self)) (defmethod (setf b) :after (val (self sum)) (declare (ignore val)) (compute self))

Content of marginal relevance to Perl: Maybe there's something out there that implements something akin to Lisp's :after methods in Perl that would do what you want. I don't know, though.