How can we dynamic activate the debugger in Perl?

while (<INPUT>)
   {
   $DB::trace = 1, next if /debug/;
   $DB::trace = 0, next if /nodebug/;
   # more codes
   }

When run under the debugger, this enables tracing when the loop encounters an input line containing "debug" and ceases tracing upon reading one containing "nodebug". We can even force the debugger to breakpoint by setting the variable $DB::single to 1, which also happens to provide a way you can debug code in BEGIN blocks (which otherwise are executed before control is given to the debugger).