Perldoc Search: "$^D" perl-5.20.1
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2 PODs, 2 LINEs found.
30 : * When the execution of your program reaches a point that can hold a breakpoint, the "DB::DB()" subroutine is called if any of the variables $DB::trace, $DB::single, or $DB::signal is true. These variables are not "local"izable. This feature is disabled when executing inside "DB::DB()", including functions called from it unless "$^D & (1<<30)" is true.
952 : $^D The current value of the debugging flags. May be read or set. Like its command-line equivalent, you can use numeric or symbolic values, eg "$^D = 10" or "$^D = "st"".