That can help, but you're probably have to redefine the builtins undef and return because tieing evrything to Undef by hand is not easier then just writing ||0 everywhere.

This, however won't help at all if BUU gets the undefs from uninitialized hash or array elements. In that case, you should probably create a tied array/hash that gives 0 for uninitialized elements. (Let me note that you can set what a hash returns instead of an uninitialized element in ruby.)