I doubt it's feasable without patching perl5db.pl.

If I were you, I'd make the breakpoint conditional from a global variable.

update

something like $DB::single = BREAK should do, like this BREAK can be either a constant or even a function.

activation of BREAK() can be done when starting up the debugger

When perl5db.pl starts, it reads an rcfile (perl5db.ini for non-interactive sessions, .perldb for interactive ones) that can set a number of options. In addition, this file may define a subroutine &afterinit that will be executed (in the debugger's context) after the debugger has initialized itself.