The general question of how to provide a legacy API to a modern system is pretty easy to answer - you'll want to look at singletons for most of your work. The $64K issue is how to provide the state your objects need so that they do what they need to do. Generally, one would hope that the mapping between the parameters passed into Legacy::Ugly_Sub_Name() and New::pretty_method() is extremely close. The only issue I've run into doing this is when the Legacy package maintained a bunch of state using file-scoped globals. If that's the case, I'd look at some sort of external datastore, like a RDBMS, especially if you're in a persistent environment.

As for specifics, the previous posters got you covered, I think.