Without specifying which, mech automatically selects the first form; you need to select the second form on that url, so:
gives me something.
form_number => 2,
sess => "summer",
course => $prefix,
UPDATE:...and that something wasn't good.
I've always found it easier to debug form automation in WWW::Mechanize by doing one thing at a time. This works for me:
my $mech = WWW::Mechanize->new(cookie_jar => undef);
$prefix = "ANT";
$mech->set_visible( [radio => 'summer'] ); # page default anyway
$mech->select( 'course', $prefix );
$mech->click_button( name => 'submit' );
- clarified first sentence.