What's the difference between a list and an array, as far as when is concerned?

This is one of the ways that given/when break expectations in that in most cases, the use of an array expression @a generates a list.

The only other exception to this I can think of is a subroutine with a prototype of (\@) which causes a reference to the array to be passed to the subroutine. I think that's what is happening here, though the documentation leave much to be desired.