You can't after the fact, but you can trigger dropping into the debugger from code if you've started the program under it to begin with.

• Make a signal handler for, say, SIGUSR1 which sets $DB::single = 1;
• Start with perl -d mydaemon regular arguments and just hit "c" when you get the debugger prompt
• When you want the program to drop into the debugger, send it your signal (kill -USR1 pid) and you should get dropped into the debugger inside your signal handler.
• Profit! Debug!