opendir my $dh, "$dir/$sub" or die "opendir $dir/$sub: $!\n";
while (defined(my $fn = readdir($dh))) {
next unless is_maildir_basename($fn);
opendir my $dh, "$dir/$sub" or die "opendir $dir/$sub: $!\n";
while (defined(my $fn = readdir($dh))) {
next unless is_maildir_basename($fn);