use IO::File;
my %Registration_Number_Files;
my $Registration_Number_Path = '/path/to/registration_numbers';
# ...
sub write_record {
   my ($country_code, $header, $record) = @_

   my $file_handle = $Registration_Number_Files{$country_code};
   unless($file_handle) {
      my $file_name = "$Registration_Number_Path/$country_code.txt";
      $file_handle =  IO::File->new(">:raw:utf8", $file_name) 
         or die "Failed to create '$file_name'";
      print $file_handle $header;
      $Registration_Number_Files{$country_code} = $file_handle;
   }
   print $file_handle $record;
}