Is it worth while to allow overriding one of the core methods? I am thinking of the ability to inject a checksum algorithm into an application that hardcodes the key.

Another thought - if overriding an existing checksuming algorithm is to be avoided at all costs, how about just failing in the case of a duplicate, and recommending the plugin module apply a namespace:

package Algorithm::CheckDigits::Plugin::MyStuff;
...
Algorithm::CheckDigits::plug_in( 'MyStuff::method1', 'My::Implementati
+on' );
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It should keep the logic simpler (die if exists...) and still allow for a mass import (as in the case of the core modules in the current implementation). The Core:: and unspecified namespace (Core::Foo and Foo) could be declared as reserved for A::CD. Enforcing that declaration is your call (sort of like not mucking around in the internal data structure of an object).

Since this is your module and my main focus is to be able to tie Algorithm::Damm into the Algorithm::CheckDigits framework, as long as that is the end result, I will defer to you, and my current holdings in round-tuits.

Note: I found a bug in my implementation where I didn't lc the key on insert. I will apply a fix for that, since it needs to happen no matter what the outcome of the plugin policy for this module ends up being. fixed.

Update: I also am concerned about the inability of being able to use the same mechanism in the core A::CD framework. It appears that this proposal would require the creation of a registry in each plugin class - at which point I might as well just subclass A::CD and be done with it, as the complexity for the user is pretty much the same. I like the simple syntax for the CheckDigits() call.