use strict;
use warnings;

my %AB;
while (<DATA>) {
     chomp;
     my ($id, $name, $desc, $app) = split /\s*,\s*/;
     $AB{$id}{app}      .= ",$app";
     $AB{$id}{name_desc} = "$name, $desc";
}

for (keys %AB) { $AB{$_}{app} =~ s/^,// } # remove leading comma

for my $id (sort keys %AB) {
    print "$id, $AB{$id}{name_desc}, $AB{$id}{app} \n";
}

__DATA__
1, XYZ, desc, PDF
1, XYZ, desc, QFZ
2, YGH, desc, LMN
[download]

Update: On second thought, this version is cleaner because it does not unnecessarily inject that comma:

my %AB;
while (<DATA>) {
     chomp;
     my ($id, $name, $desc, $app) = split /\s*,\s*/;
     push @{$AB{$id}{app}}, $app;
     $AB{$id}{name_desc} = "$name, $desc";
}

for my $id (sort keys %AB) {
    print "$id, $AB{$id}{name_desc}, ", join(',', @{$AB{$id}{app}}), "
+\n";
}
[download]

This prints:

1, XYZ, desc, PDF,QFZ
2, YGH, desc, LMN
[download]

I do know what I've been told about using the "my" before the various scalars, arrays, hashes, etc. The person that I'm writting these for doesn't like when I use those.