opendir(D, "unfinished/") or die "unable to open unfinished/: $!\n";
my @files = readdir(D);
closedir(D);
for (@files) {
  next unless ($file =~ m/^(\d\d\d\d)-(\d\d)-(\d\d)$/);
  my ($year, $month, $day) = ($1, $2, $3);
  eval {
      ... process download for $year/$month/$day ...
      ... in case of an error, raise an exception with die ...
  };
  if ($@) {
    warn "downloading for $year/$month/$date failed: $@\n";
  } else {
    rename("unfinished/$file", "completed/$file");
  }
}