How to specify a "_p_void" value for Sax2XMLReader::setProperty?

I am trying to set an external schema location for a Sax2XMLReader 
parser. According to the "SAX2 Programming Guide" the syntax should be 
similar to the following:

$parser->setProperty(
  "http://apache.org/xml/properties/schema/external-schemaLocation",
  "http://www.example.com file_name.xsd");

Any way I try it, I get the error message

Error in eval: Type error in argument 3 of SAX2XMLReader_setProperty.
Expected _p_void at validationtest.pl line 45.

What is the perl equivalent of a _p_void? Am I missing something? Or is 
it a bug? Thanks for any ideas,

Martin