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))
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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.