It looks like you are trying to post an XML::Writer object to a URL. Without looking at the API for either XML::Writer or LWP::UserAgent I can only guess, but I seriously, seriously doubt that procedure is possible. What you will want to post is not the writer object, but the output it produces.
Re^3: HTTP Post of XML data
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I think he's trying to use Perl 5.8's "stringy filehandles".
From the docs:
An object blessed into IO::Handle or one of its subclasses (such a
+s IO::File), or a reference to a string; if this parameter is not pre
+sent, the module will write to standard output. If a string reference
+ is passed, it will capture the generated XML (as a string; to get by
+tes use the Encode module).
But I am getting an error, I think may be java in the jboss log.
14 Nov 2007 00:15:56,547 INFO [STDOUT] REQUEST RECEIVED BY SERVLET:<m
+anager xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" >
14 Nov 2007 00:15:56,547 ERROR [STDERR] [Fatal Error] :1:69: XML docum
+ent structures must start and end within the same entity.