Well, all your points are well taken. Obviously you should always use the right tool for the job. But I am actually not sure that Moose is the wrong tool for you. Because Moose uses metaclasses to build all instances, and those same metaclasses also build the accessors, there is great opportunity for optimizations here.

As I mentioned, Moose uses Class::MOP, which builds blessed HASH based instances, but this is just the default. It is possible to extend Class::MOP to build other kinds of instances as well. In the examples for Class::MOP, I show how it can be used to build inside-out classes. I also have an example of a Lazy class which will not initialize it's fields until the absolute last possible moment. I have been considering an ARRAY based example as well, but haven't gotten around to it. There is also nothing to stop you from an Inline::C based version which possibly could be made even more space/time efficient than an array version.

As for how all this will work with Moose, allow me to explan. The primary role of Moose is to collect meta-data about your class definition, and create a number of metaobjects from it. (The extra overhead of these metaobjects will usually be fairly small, since they are a per-class cost, and not a per-object/instance cost, and most systems will have a reasonably small amount of classes compared to the amount of object/instances they generate. But I digress here ...). These Moose metaobjects are used to build things like accessors, and eventually to help build the actual instances too. Since Moose only makes you pay for the features you use, if, for instance, you don't choose to use a type constraint, you don't pay for it's overhead. Now, since all the details of your class is stored as meta-data by Moose, and the Moose metaobjects are managing the details of your object instance structure and means of accessing it (through accessors), it is possible to swap a different Class::MOP based engine into Moose and have it create ARRAY based instances without having to change the surface syntax of your Moose based classes (assuming you don't break encapsulation that is).

Now, the example I describe is not easily accomplished at this point, because I have not added the proper hooks into Moose for his kind of thing. But Class::MOP has been designed to do this kind of thing from the very start, so its just a matter of getting the tuits to do it.

Moose and Class::MOP are tools designed to work with Perl 5's OO system and not against it. This means that they should not get in you way if you need/want to do something different, because after all, TIMTOWTDI :)