Sure. LWP can do this for you.

my $req = HTTP::Request->new( POST => 'http://some.site/app' );
   $req->content_type( 'text/xml' );
   $req->content( $myxml );
...
[download]

It's up to the server side on how to interpret the data coming to them. They may or may not need the content type set to 'text/xml'. If they're using CGI, you need to set the content_type to something other than 'application/x-www-form-urlencoded' or 'multipart/form-data'. In this example, by setting the content_type to 'text/xml', CGI will not try to parse the incoming stream (See the section 'HANDLING NON-URLENCODED ARGUMENTS' in CGI). But ... if they used something other than perl or CGI, they should tell you what the content_type should be.

Update I probably wasn't clear. There is no inherent tie between a POST and forms. Forms are normally POSTed (but they don't need to be) and POSTs are normally handed off to form processors. But that's just convention. Just as you can use a GET to send form data (not that I recommend it), you can use a POST to do thinks other than form processing.