perrin's class factory suggestion sounds good, as long as the behavior changes globally. For per-object changes something like Class::MethodMaker's method proxying may be more appropriate. Basically you setup a normal composition relationship (StoneGatherer has a Gatherer) but the particular Gatherer object is held in an attribute. Then you define the list of methods that should be proxied to the Gatherer that's actually in use.

This is something like the system used by Krang to manage the relationship between elements and element classes. Each element is assigned to an element class at runtime and the object behaves as though the element class is its base class via proxying. For example, this code:

   $element->publish();
[download]

Is really:

   $element->class->publish(element => $element);
[download]

-sam