You could manipulate &DB::sub.

I'd probably install a wrapper which only calls the original code if my conditions are met or manipulates $DB::deep before calling.*

See http://search.cpan.org/dist/perl-5.8.7/lib/perl5db.pl#sub

and perldebguts#Writing-Your-Own-Debugger

update

*) alternatively setting $DB::single to 2 corresponds to the n cmd. Use local to avoid global effects.