Code:
  my $ua  = LWP::UserAgent->new;
  my $res = $ua->request(POST 'http://myserver.com/LLS/GenPublishMessa
+ge',
                         Content_Type => 'multipart/form-data',
                         [xmlmsg => $xmlmsg]);

Response:
HTTP/1.1 302 (Found) redirecting for authentication purposes
Connection: close
Date: Wed, 31 Jul 2002 21:23:53 GMT
Location: http://myserver.com/securecontrol/ct_logon_int_qa.html?ct_or
+ig_uri=%2FLLS%2FGenPublishMessage
Server: Netscape-Enterprise/4.1
Client-Date: Wed, 31 Jul 2002 21:23:53 GMT
Client-Response-Num: 1
P3P: CP="NON CURa ADMa DEVa TAIa IVAa OUR DELa SAMa LEG UNI PRE"
[download]

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