#!/usr/bin/perl

use strict;

## become a daemon
exit if fork();

## loop and look for a filename appearing
my $fn="/homes/bob/important_file.txt";

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

## never here!
exit;