tie %Hash, 'DB_File', $path_to_hash, O_RDWR|O_CREAT, 0666 or die "Can't tie $path_to_hash: $!";

tie %Hash_values, 'DB_File', $path_to_hash_values, O_RDWR|O_CREAT, 0666 or die "Can't tie $path_to_hash_values: $!";

while (($key, $value) = each (%Hash)) {
    $Hash_values{$value} = $key;    # Or any value for $key
   }

untie %Hash;
untie %Hash_values;