my $data = <<EOD;
Student Id Class John 23 2 Mark 33 3 Jerry 44 2 Sam 55 2 Tom 65 3 Teststud 28 2
EOD

my %h;
my @arr = split /\s+/, $data ;
for (my $c=3; $c<($#arr);$c+=3) {
  push @{$h{$arr[$c+2]}}, $arr[$c];
}
print $_."=".join(",",@{$h{$_}})."\n" for (keys %h);
##</code><code>##

3=Mark,Tom
2=John,Jerry,Sam,Teststud