...
my %courses;
for my $row (@results) {
    my $course_id = $row->[0];
    $courses{ $course_id } ||= {
        ...
        desc => $row->[1],
        sittings => {},
        ...
    };
    $courses{ $course_id }->{ sittings }->{ $row->[2] } = $row->[3];
};