use warnings;
use strict;
use Data::Dumper;
$Data::Dumper::Sortkeys = 1;

my %courses;
while (<DATA>) {
    chomp;
    my ($id, $desc, $sit, $date)    = split /,/;
    $courses{$id}{desc}             = $desc;
    $courses{$id}{sittings}{$sit}   = $date;
}

print Dumper(\%courses);

__DATA__
10,Programming 101,1,01-SEP-2011
20,Databases 101,1,03-SEP-2011
10,Programming 101,2,20-SEP-2011
20,Databases 101,2,27-SEP-2011