Just put your additional arguments into the request:

my $req = POST $url, Content_Type => 'form-data',
            Content        => [
                    loaderid   => $loaderid,
                    zipped     => $zipped,
                    user       => $user,
                    filetype   => $filetype,
                    file       => [ $fname ]
                                        %preferences,
                ];
[download]