Even though your question is already answered, here's a different approach.

my $hr_data = $dbh->selectall_hashref(
    "select
        course_id,
        course_name,
        course_sitting_id,
        course_sitting_date 
    from courses", 
    ["course_id", "course_sitting_id"]) or die $dbh->errstr;
[download]

This gets you the following structure:

{
    '10' => {
        '1' => {
            course_id => '10',
            course_name => 'Programming 101',
            course_sitting_id => '1',
            course_sitting_date => '01-SEP-2011'
        },
        '2' => {
            course_id => '10',
            course_name => 'Programming 101',
            course_sitting_id => '2',
            course_sitting_date => '20-SEP-2011'
        },
    },
    '20' => {
        '1' => {
            course_id => '20',
            course_name => 'Databases 101',
            course_sitting_id => '1',
            course_sitting_date => '03-SEP-2011'
        },
        '2' => {
            course_id => '20',
            course_name => 'Databases 101',
            course_sitting_id => '2',
            course_sitting_date => '27-SEP-2011'
        },
     },
};
[download]

From here you can mold the hashref to your preference.