Update: I looked closer at your code and the first issue is that you're never sending the filled out form back to the server:

# Update Form
$forms[1]->click("update");
[download]

Use something like the following code, as suggested by the HTML::Form documentation:

my $filled_out_request = $forms[1]->click;
print $filled_out_request->as_string; # for debugging, see below
$response = $ua->request($filled_out_request);
[download]

Whenever automating access to a webpage, it is most important to replicate what the browser is sending. There are many tools nowadays to do that:

• HTTP::Recorder - it is a proxy that sits between your browser and the web and it will create a WWW::Mechanize script that replicates all your actions.
• For Mozilla there is the Live HTTP Headers extension, which displays all data that is sent in a log file against which you can compare your programs output
• My own module, Sniffer::HTTP, which also can display all HTTP traffic as it gets sent and received.

With any of these tools, the process is always to

1. Get the original data sent by the browser
2. Write a program to replicate it
3. Compare the data sent by your program against the original data
4. Repeat

Of course, I, as the author, think that my tools (Sniffer::HTTP and HTTP::Request::FromTemplate) are superior, but I use the other tools as well.