perl5db.pl POD — the most detailed POD documentation about the debugger
or you can read the actual perl5db.pl source code, though be warned the code is definitely dense and inscrutable
DB.pm — programmatic interface to the Perl debugger
Note that it has lagged behind the newest perl5db.pl functionality. The documentation says it is "Missing some of the later additions to perl's debugging functionality. As such, this interface should be considered highly experimental and subject to change." The file's version history indicates it has had no modification of any significance since Feb 7, 1999. That date corresponds roughly to perl5db.pl v1.0402, and there have been very significant expansions to perl5db.pl since that version.