use HTTP::Request::Common qw/POST/;
[...]
my @values=("$directory\\".$snapshots[$i], "$directory\\".$thumbs[$i]);

$req = POST 'http://bigpichost.de/upload.php',
       Content_Type => 'form-data',
       Content => [
           map { ('userfile[]' => [ $_ ]) } @values
       ];
$cookie_jar->add_cookie_header($req);

$res = $ua->request($req);
[...]