Data::Mapper ver. 0.06 has been released today.
This release includes an important feature implemented by トミールさん; Data class now can be also a POPO, that is, you can re-use your own hash-based object and map data from external storage/API to it. In addition, you can customize what class to be used as data class by overriding Data::Mapper#data_class method. It's useful when your want to locate your data class somewhere against where the module sets by default.
I think Data::Mapper is almost ready for use. I actually use it for my job (but not in production yet). If you have any idea/requirements, please send me feedback!