Most of the time, I write straightforward Perl OO code using blessed hashrefs and cloning of objects when new() is called on prototypes. Add some AUTOLOAD from a fixed set of attributes (and run-time method generation for speedup).
I try to keep everything nice and subclassable, but when that just wouldn't work, I offer some extension mechanism via delegation like Math::Symbolic::Custom. Also, I tend to offer two interfaces. One kept as simple as possible for the general usage case and one involving things like closures for the ultimate control in pathological cases.