I do not believe perl literally puts all 1,000 elements of longarray on the stack and then pops it, as C would do.

Read the source code; the relevant function is pp_aassign in pp_hot.c.

when I tried it, call(@longarray) and call(\@longarray) took almost the same amount of time.

That doesn't mean anything; wallclock is the second worst way to benchmark anything. It's too fragile and you can't trust it.

I do not understand what the purpose of a list is.

A list is a sequence of zero or more expressions. Without a list... well, you'd have no program.

if sub's returned arrays, then they could easily be assigned back to arrays

If Perl 5 subs returned arrays, not lists, you'd need special syntax (like function return slicing) to do anything else than assign to arrays. You'd also have to figure out how to return a list of arrays; you'd have to add some sort of literal tuple as a first-class data structure, and then you get into the complexity of fixed arity versus variable arity, and ... well, Perl 5 has its flaws, but I'm not sure you gain a lot making this more complex here.