...
# do they provide a scalar ref as output buffer?
if ($output_ref) {
  $fh_out = new IO::Scalar($output_ref);
}
...
if (defined $fh_out) {
  # write XML to the scalar-tied file handle
  $self->{x_out} = new XML::Writer(OUTPUT => $fh_out);
} else {
  # default: print to STDOUT
  $self->{x_out} = new XML::Writer();
}
...