It gets more confusing (but then it gets more clear).

@foo is an array

Yes.

while (1,2,3) is a list

1, 2, 3 is a list. The parentheses have nothing to do with its listishness. The parentheses are there only so that the parser doesn't parse the whole expression as:

(@foo = 1), 2, 3;

... as it would if there were no grouping parentheses. They indicate which list you want to assign.