What content you should put in $content depends very much on what content $url is expecting to receive.

The most commonly accepted format for the content is called "application/x-www-form-urlencoded" (them HTTP guys sure know how to choose good names for things). This is a string formatted like this:

key1=value1&key2=value2
[download]

Luckily, WWW::Mechanize (or rather, one of its dependencies, HTTP::Request::Common) will automatically build that string for you given an arrayref. So you could set $content like this:

 $content = [
   key1   => 'value1',
   key2   => 'value2',
   ];
[download]

Content-types other than application/x-www-form-urlencoded are certainly possible, but unless you've been specifically told to use a particular content-type, it's pretty safe to assume that application/x-www-form-urlencoded is what is expected.