Putting the line kill 2, $$ will cause the debugger to break at that point in the source (on Unix, anyway).

I had never seen that trick before. Cool. I have been using this one instead:

$DB::single = 1;
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That causes the debugger to enter "single-step" mode, which amounts to programmatically setting a breakpoint in the next executable line. This is particularly useful for causing the debugger to stop at places that happen before the first executable line, e.g.:

BEGIN {
  $DB::single = 1;
}
use Foo;
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The code above will enable stepping through the loading of 'Foo', which otherwise would happen before the place at which DB normally starts (i.e. the first executable line). I see that kill 2, $$ works well for this too. One more trick for the bag.