BEGIN {
   use Benchmark;
   my ($oldb, $newb);    # inside BEGIN for closure

   sub mybench {
      return unless ($opt_b);

      $newb = new Benchmark;
      if ($oldb) {       # i.e. don't run first time through
          print timediff($oldb, $newb);
      }
      $oldb = $newb;
   }
}

Getopt::Std;

getopts('b');

mybench();
# some code here
mybench();
# some more code here
mybench();