undef is a special value in the scheme I've proposed. All results are taken in scalar context (assigned to $_), since this is a line-piping scheme. If the grep step returns undef, the iterator goes back to the first sub to try a new line. undefs do not show up in the output stream.

You could use an empty list as your no-line-returned indicator, but it would require you evaluating the subs in list context and then turning the returned value into a scalar afterward. That would be problematic for some common pipe actions, like reading one line at a time from a file (in list context, the whole file would be read).