It doesn't delay creation of the list and undefined (or false) items in the list can break things like while (defined $result->next).

That's why you make your iterator return an empty list when you reach the end of its list and then test using while (my ($item) = $result->next).

While I like the concept, I'm not partial to the name. Sub::Context is not much better, though slightly more descriptive; I have very little idea what to expect by just seeing that name. Sub::ContextAware is very close, but I find it slightly misleading as well, so I propose Sub::ContextAwareness. It would be even better to work "Default" into the name somewhere, but that'd get unwieldly for not too much gain.