not sure what the issue is, but maybe try WWW::Mechanize's submit_form method instead?

my $mech = WWW::Mechanize->new();
$mech->get("https://www.site.com/forums/cs/dispatch.cgi/_admin/diskUsa
+geForm");
$mech->submit_form(
  form_name => 'CFO',
  fields => {
    pforumClass => '__all__',
    tforumClass => '__all__',
    uforumClass => '__all__',
  },
);
[download]

Also maybe see if the output from mech-dump gives you any extra information ...

And also do a die $mech->content; after the ->get() to make sure it's really the page you expect it is ...