Thank you for the concise and clear explanation. But ... :)

Isn't the 'nulltime' compensation code meant to correct for (most of) that?

  # "Null time" compensation code
  $nulltime = 0;
  for (1..100) {
    my($u,$s,$cu,$cs) = times;
    $cstart = $u+$s+$cu+$cs;
    $start = Time::HiRes::time;
    &$testDB;
    ($u,$s,$cu,$cs) = times;
    $cdone = $u+$s+$cu+$cs;
    $done = Time::HiRes::time;
    $diff = $done - $start;
    $nulltime += $diff;
  }
  $nulltime /= 100;
...

  my($delta);
  $delta = $done - $start;
  $delta = ($delta > $nulltime) ? $delta - $nulltime : 0;
...
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