my $response = $browser->post($url, Content => join('', @data));
[download]

... and then read up on the fat comma (=>) operator in perlop.

What you're doing is equivalent to:

  my $response = $browser->post($url, 
    Content  => $data[0],
    $data[1] => $data[2],
    $data[3] => $data[4],
    ...
  );
[download]

... which is not expected by the post method.

Update: Oops, thanks Jenda :-/

-David