As mentioned, you simply need to figure out what the server is doing. Live HTTP Headers is a good way.

The following just selects Accounting courses:

my $ua = WWW::Mechanize->new;
my $res = $ua->post( 
    'http://students.yale.edu/oci/resultWindow.jsp', 
    Content => 'term=200901&GUPgroup=A&CourseNumber=&ProgramSubject=AC
+CT&InstructorName=&timeRangeFrom=08&timeRangeTo=21&ExactWordPhrase=&y
+cRules=new&distributionGroupOperator=AND&Submit.x=145&Submit.y=7'
);
$ua->follow_link( tag => 'frame', name => 'resultFrame' );
$ua->follow_link( tag => 'frame', name => 'resultList' );
print $ua->content;
[download]

Now you can parse the html to get at the information; it's the second table in the body.