I think it's great to see people daring to dive into the unplumbed depths of perl5db.pl. Few who have ventured there remained sane enough to describe their experiences afterwards.

I think the likely response from p5p will to that you cannot change the behaviour of n for backwards compatibility reasons. The fact that you can no longer step through the innards of a map or grep is too much of loss. That said, I think the concept has great merit. To that end, an approach that is more likely to succeed would be to introduce a new command letter. N is currently free (according to my copy of bleadperl).

As a minor stylistic issue, the Perl core hasn't drunk the PBP kool-aid, and thus you would probably want to write your check as the more compact

You also don't need to capture it. I also wonder if that will pick up a "map" in a quoted string or a comment...

In a similar vein, I think my @sublast = split m{-}xms, $sub{$sub} looks slightly silly.

split /-/ is fine.

update: I am a bit thick sometimes. It took tye's reply for me to realise that there is no loss of functionality. So the N hack isn't necessary. So what's left is purely a matter of code style.