Here's a snippet from an upload script I use. I can't remember where I found the docs that explain what to set here though. I will look.

$response = $ua->request(POST $url, Content_Type => 'form-data',
                         Content => [ TextField1 => "TEST", mptest => 
+[$file]]);
[download]

$file is just the name of the file but I seem to recall that it can also be a filehandle.