Found the answer at http://groups.google.com/groups?q=HTTP::Request::Common+upload&hl=en&rnum=1&selm=m3snel5r7a.fsf%40ActiveState.com

   my $ua = LWP::UserAgent->new();

     my $req = POST $url,
        Content_Type => 'form-data',
        Content      => [blah => $something,
                         file => [undef,
                                  'foo.txt',
                                   content_type => 'text/plain',
                                   content => $x
                                 ]
                        ];
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