Hi,
I finally found the solution to my problem.Just thought that I'd share it with others...Here's how u post multipart/form-data using HTTP::Request::Common, without having to create a file containing the data (submitting the content directly ) :

  use HTTP::Request::Common;
        my $ua = new LWP::UserAgent;
        my $res = $ua->request(GET 'http://2.2.2.2/page.asp');

        if ($res->is_success) {
    {
        no strict 'subs';
        $res = $ua->request(POST 'http://2.2.2.2/page.asp',
                Content_Type => 'form-data',
                Content => [
                    username => test,
                    password => test,
                    init => [undef, 
                             "filename",
                              content_type => 'text/xml',
                              content => $request_xml,],
                    ]
                );
    }
    ...
[download]

Hope this helps someone... :)