I'll start by saying that I expect Moose to be ubiquitous. If it's not now, it will be. Also, anyone installing something as obscure as Array::GroupBy is sure to have something as mainstream as Moose.
Anyway, what I got in this particular case was mostly Moose::Util::TypeConstraints. At object creation, and any time an attribute is set, the attributes are checked against the constraints I set. If I weren't using Moose, I'd be using Params::Validate. Aside from that, I get the usual OO framework stuff: a constructor and attribute accessors.
The make-work code I'm saved from writing is, again, mostly the input validation stuff. My version of Moose is a ways behind the current, so I wound up doing some of this myself in order to get the detail I wanted. I found it pretty tedious, and the results challenge comprehension. With a newer version of Moose, this was easier to do and easier to read. Without Moose (or some module), it could have been more work than writing the actual functionality of the module.