Module::Pluggable is a wonderful module. I use it on a regular basis. But it does not do what the OP is looking for, at least in the general case.

Module::Pluggable would not, for example, find the Circle, Square, and Line subclasses in the OP's code, as they are contained within the same source file.

Module::Pluggable does its thing based on the module file names and their implied namespaces. It knows nothing about the actual inheritance relationships in the code. If you were to send it to find things under "Element", it would return packages found in the file Element/NotAChildClass.pm while missing an Element::Pentagon class that lived in the file SomewhereElse/Pentagon.pm.