Well ... you give us no code nor info on frameworks, modules, client set headers, http method, etc. I'll just assume you're POSTing XML with a mime type of application/xml and using CGI on the server side. If those assumptions are correct, then you just need to:

my $data = $query->param('POSTDATA');
[download]

If those assumptions are not correct, you're going to need to provide more information because there are so many different ways for a client to send data and a server to process requests.