If you're on a *NIX box, take a look at cron (man cron)...

NAME
     cron - clock daemon

SYNOPSIS
     /usr/sbin/cron

DESCRIPTION
     The cron command starts a process that executes commands  at
     specified  dates and times. Regularly scheduled commands can
     be specified according  to  instructions  found  in  crontab
     files  in  the directory /var/spool/cron/crontabs. Users can
     submit their own crontab file using the crontab(1)  command.
     Commands which are to be executed only once may be submitted
     using the at(1) command.
[download]

I'm sure there is a similar function under Win32, but unfortunately I have no idea what it would be. Something I've found useful for reloading scripts while they're still running (it leaves zombie processes, but it works for a quick hack. If anyone can suggest something better, I'd be interested in knowing!)

if ($cmd eq 'reload') {
  my $child = fork ();
  if ($child) {
    print "Reloading...\n";
    exec ('perl script.pl');
    exit 1;
  } else {
    exit 1;
  }
}
[download]

And if you're interested in reloading libraries, you may want to take a look at this node.

Hope it helps somewhat.

scott.

Edited: added line about reloading libraries.