open( my $fh, $infile ) or die "Cannot open '$infile' for reading: $!\
+n";
my %data;
while ( defined( $_ = <$fh> ) ) {
    chomp;
    my @line = split( $_, ',', 2 );
    push @{$data{$line[0]}}, $line[1];
}
close( $fh );

open( $fh, ">$outfile" ) or die "Cannot open '$outfile' for writing: $
+!\n";
foreach my $k ( sort keys %data ) {
    print $fh join( ',', $k, @{$data{$k}} ), $/;
}
close( $fh );
[download]

Now, if this is homework, I wouldn't turn that in - it'll be obvious you got help. If it's not homework, take the time to figure out what I did. The key is push @{$data{$line[0]}}, $line[1];.