mr.nick: shouldn't

while (1) {
   if (-f $fn) {
       ## a file was found, do something
       DoSomething();
       ## then exit
       exit;
   }
   ## pause 5 minutes before checking again
   sleep 300;
}
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really be:

while (1) {
   if (-f $fn) {
       ## a file was found, do something
       DoSomething();
       ## then break the loop
       break; ## <<-- not exit();
   }
   ## pause 5 minutes before checking again
   sleep 300;
}
[download]