Including "$!" as part of that first error message is useless. "$!" is only for system error messages, like why you can't open a file and such. Also, as long as the purpose of your function is to write filenames, there's no point in saving them in an array (IMO):

sub indexfiles {
  my $cod = shift || die "Must supply filename!";
  my @dirs = @_ ? @_ : ".";

  open(DB, ">$dbpath/$cod.txt") or die "Error: $!";
  my $fh = select(DB);
  local $\ = "\n";
  find( sub {
    local $_ = $_;
    print if s/\.mp3$//;
  }, @dirs);
  close DB;
  select($fh);
}
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ooo  O\O  ooo tilly was here :,(