use Time::HiRes qw( time sleep );

use constant {
   TIMES_PER_PERIOD => 3,    # Integer
   PERIOD           => 1.0,  # Fractional seconds
};

my @times = (0) x TIMES_PER_PERIOD;
while (my $r = get_request()) {
   my $wait = ($times[0] + PERIOD) - time();
   if ($wait > 0) {
      sleep($wait);
      redo;  # In case sleep got interrupted.
   }

   do_request($r);
   shift(@times);
   push(@times, time());
}