I ran cpan_grep.pl using the search
/[*\@\$\%]DB::/, to find modules that interact with perl5db.pl. The results were... voluminous. But interesting too.
The vast majority of the references to the DB package were:
$DB::single = 1to allow the module's author to start the Perl debugger, hit 'r'un, and for the debugger to automatically stop at that line
@DB::args— TODO: figure out what this means
TODO: continue processing the results