In the later case, to calculate the CPU usage ratio, you can use something like that:

my ($cpu_time, $clock_time) = (0, 0);

sub your_event_processing {
  my ($start_user, $start_system) = times;
  my $start_clock = time;

  # do the event processing here
  # ...

  my ($end_user,$end_system) = times;
  my $end_clock = time;

  $clock_time += ($end_clock - $start_clock);
  $cpu_time += ($end_user - $start_user + end_system - $start_system);
}

END {
  printf STDERR "clock_time: %d, cpu_time: %d (%4.2f%%)\n",
                $clock_time, $cpu_time, $cpu_time * 100 / $clock_time;
}
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And let it run for some hundreds of events, so that the rounding errors go away.