iteration form of map

The 5.20 pairlist slices make me eager for more good stuff.

I'd really like to see something that allows transforming a list like
map, but working on an iterator instead of a list.  Perhaps something
like:

    iter BLOCK EXPR

equivalent to

    do { my @list; local $_; while(defined($_=EXPR)) { push @list, do
BLOCK } @list }

only using the stack.

(I have no idea what perl6 provides in this area.)