A lot depends on what kind of commonalities you have. If significant portions of the

• interface are similar, you don't have inheritance - you have an API
• functionality are similar, you want to have a utilities module
• functionality and interface are similar, you should consider inheritance from some common base class(es)

A utilities module shouldn't contain any references to the innards of any object. It should literally be a bunch of subs along the lines of:

sub min { $_[0] < $_[1] ? $_[0] : $_[1] }
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These are subs that do something useful, but are completely atomic - they don't have any reference points outside themselves.

Now, you mention listcmd and browsecmd. I don't know Tk, but it sounds like they both deal with commands, in some sense. So, maybe you don't actually have inheritance as much as delegation. Have you considered abstracting out some cmd object and having listcmd and browsecmd both contain an instance of cmd?

From what I'm guessing, the list part would be inheritable. So, let's say you had a listfoo and a listbar. You would have a list baseclass that both would inherit from, plus foo and bar classes that they would delegate to.

DOes that make sense?