Ergo, they indicate a list.

No, they group expressions and change precedence. The list is there regardless of the presence or absence of parentheses. The parentheses only change the order in which Perl will evaluate the comma-separated expressions.

Yes, the comma operator provides the list context....

No, the assignment to the hash provides the list context. That's why you can write:

... and get a warning about an odd number of elements to a hash. No parentheses. No commas. Still a list.