I think what you want is to pass to your content method an array reference, not a string. In your first example,  $req->content("xmlmsg=$xmlmsg"); should be $req->content([ xmlmsg => $xmlmsg ]);.

The pertinent part of the man page I was referring to is:

The POST method also supports the multipart/form-data content used for Form-based File Upload as specified in RFC 1867. You trigger this content format by specifying a content type of 'form-data' as one of the request headers. If one of the values in the $form_ref is an array reference, then it is treated as a file part specification with the following interpretation:

           [ $file, $filename, Header => Value... ]
[download]

The first value in the array ($file) is the name of a file to open. This file will be read and its content placed in the request. The routine will croak if the file can't be opened. Use an undef as $file value if you want to specify the content directly. The $filename is the filename to report in the request. If this value is undefined, then the basename of the $file will be used. You can specify an empty string as $filename if you don't want any filename in the request.

Note that I have not tested this code, but I hope it points you in the right direction.
Cheers,
Shendal