it'd be nice to have some sort of iteration interface so that we could use for against a collection or generator or iterator or array without having to flatten the values into a list.

Um, neither for my $item ( @array ) { ... } nor ... for @array; flatten?

And for hashes, use ... while (key, value) = each %hash;

Sure, Perl has special ops for non-flattening iteration of arrays and hashes, but it doesn't have an iteration interface so you can't write your own non-flattening iterator.

I guess I am still misunderstanding you because it quite easy and very common to use a sub ref closing over a counter as an iterator for use with while. By way of example, see Algorithm::Combinatorics

I'm not seeing the difficulty you are alluding to, so perhaps you could clarify for me?

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So don't use foreach. There are other non-flattening interfaces.