:hireswallclock
   If the Time::HiRes module has been installed, you can specify the
   special tag ":hireswallclock" for Benchmark (if Time::HiRes is not
   available, the tag will be silently ignored). This tag will cause the
   wallclock time to be measured in microseconds, instead of integer
   seconds. Note though that the speed computations are still conducted in
   CPU time, not wallclock time.