> Why don't you want to add the $DB::single line?
For all the same reasons the debugger supports adding breakpoints while debugging.

Also, many times when I'm debugging is when I decide where I want to add a breakpoint. I don't want to have to restart the debugger and lose my position in the executing program's call stack just to add a break point. That's annoying.

Adding a $DB::single=1 code works, but it's far from ideal.