Thank you for your answer.
I meant I waited too much to finish it. Additionally,
my $start_time = [Time::HiRes::gettimeofday()];
my ($user, $system, $child_user, $child_system) = times;
print "wall clock time was ", Time::HiRes::tv_interval($start_time), "
"user time for $$ was $user\n",
"system time for $$ was $system\n",
"user time for all children was $child_user\n",
"system time for all children was $child_system\n";
gives different results with respect to benchmark.
Sorry for any foolish comments.