You will have to learn about HTTP and HTML, and how the two interact. The URL you linked contains frames. Until you learn about frames, what they mean for HTTP and HTML, and how WWW::Mechanize treats them, you will encounter the same problem over and over.

I recommend that you start out by printing the content of what WWW::Mechanize sees as the current page:

print $mech->content
[download]

After you've seen the content and understood what the content means (which is a HTML question outside of the scope of this website), you will understand the steps necessary to get to the correct page.