Is it also the module author's responsibility to not break multiple inheritance?

The way that you described doing things underscores my point. If your funky class is in a later chain of inheritance with multiple inheritance, then you just broke can! Exactly what you said that it was your responsibility not to do!

Depending on how yhou implemented it, you may have broken can with single inheritance as well. (Particularly if the subclass tries to use AUTOLOAD. But one could argue that it is that subclass' responsibility to understand the class that it is overriding.)

Which is one of the reasons why I kept on saying UNIVERSAL::can. Overriding can in any other location leads to potential breakage. Personally I'm OK with that as long as you're clear on what breaks, and why. (I'm not a big fan of multiple inheritance, so I don't grieve when it dies. Others might.)

The dominant reason why I said it that way was to keep people from having to figure out whether I was talking about can as a noun or a verb in the same sentences. However the other issue was in the can of worms that I referred to if you try to override can locally. (Hrm, make that a verb or which noun? :-)