My question came from looking at the code shown, where the interface given only contained some 'get' methods. There weren't even any (data) references that could be indirectly mutated via that interface.

Moose's obsession with attributes seems to be a result of it trying to emulate CLOS. Classes in CLOS are essentially structs that just contain data. The behaviours are defined in multimethods that live outside of classes. Moose doesn't provide the generic function / multimethod part of CLOS, so we're just left with trying to blend the Class-is-a-struct idea into Perl (which I agree is quite a suboptimal blend :)