use File::CounterFile;
my $counter = File::CounterFile->new("/path/to/sentinel.txt", "0000");
chdir "/path/to/where/the/files/are" or die "chdir: $!";
## see if a new file has arrived
while (-e (my $file = "elmo$counter.txt")) {
  process_file($file);
  $counter = "0000" if ++$counter == "10000";
}