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
[download]

prints...

$VAR1 = {
          '10' => {
                    'desc' => 'Programming 101',
                    'sittings' => {
                                    '1' => '01-SEP-2011',
                                    '2' => '20-SEP-2011'
                                  }
                  },
          '20' => {
                    'desc' => 'Databases 101',
                    'sittings' => {
                                    '1' => '03-SEP-2011',
                                    '2' => '27-SEP-2011'
                                  }
                  }
        };
[download]

• split
• perldsc