For now, it is not of that matter, because I timed it that the script needs 10minutes to run. Later on, this will come into matter, as the script will grow larger.

You could time the function call:

while(1) {
  my $startingtime = time;
  expensive_function();
  sleep $startingtime - time() + 7200;
}
[download]

This may still introduce drift over time as time only has a granularity of one second, though.

In any case, using cron (as suggested above) is definitely the right thing to do. Just ask your friendly local sysadmin.