has base_file => (
    is       => 'rw',
    required => 1,

    #allow external names to be different from class attribute
    init_arg => 'base',
    builder  => sub {
        my $base_fh = IO::File->new( $_[0], '<' ) or die "$_[0]: $!";
        $base_fh->binmode(":utf8");
        return $base_fh;
      }
);