In those three examples you linked to, right before you say <DATA> you do $/='';, which enables "paragraph mode", it's as if the input record separator $/ was /\n\n+/.

So you are right: I had to pass an handle not an iterator (?) like <DATA>

<DATA> is the equivalent of readline(DATA), and since readline is being called in list context, it'll read all the records from the handle and return a list of them. So as long as your __DATA__ section doesn't contain any empty lines, it's essentially the same as a slurp - this is probably why the "wrong form" still works.