...much of the benefits with using objects will disepear.

You're really not losing the benefits of OO just because you have to store the private data in a complex data structure. Some of the benefits of OO are data encapsulation and data hiding, inheritance and polymorphism, and simply being able to provide an interface to your data.

To elaborate on what jasonk already said, you may be putting your data all in a hash, but typically your object will have accessors and mutators (getters and setters) which will hide that hash from the user of the object. So instead of the user doing something like $object->{private_array}[5], your class would probably provide an easier and more abstracted way of getting that item, such as $object->get_item(5). This also gives you the freedom to change the underlying data structure in the future without the interface changing.