5,#,A
1,5,B
4,5,C
2,4,D
[download]

A
B,A
C,A
D,C,A
[download]

B,A
D,C,A
C,A
A
[download]

Hmm, a requirements change :)

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1227383

use strict;
use warnings;

local $_ = join '', <DATA>;

for my $id ( /^(\d+)/gm )
  {
  print /^$id,.*,(\S+)/m;
  $id = $1, print ',', /^$id,.*,(\S+)/m while /^$id,(\d+)/m;
  print "\n";
  }

__DATA__
5,#,A
1,5,B
4,5,C
2,4,D
[download]

Hi ovedpo15,

The only problem to keep the original order of the input is that the data structure used in my solution, a hash, does not have a defined order. However, the solution is fairly easy: we only need to keep track of the original order one way or another.

One possible way to do that is to add an array (@ranks) as an auxiliary data structure when reading the file and then to loop over the array when preparing the output:

use strict;
use warnings;
use feature 'say';

my (%tree, @ranks);
while (<DATA>) {
    chomp;
    my ($id, $pid, $name) = split /,/;
    $tree{$id} = {pid => $pid, name => $name};
    push @ranks, $id;
}
for my $id (@ranks) {
    my @parent_list;
    my $temp_id = $id;
    while (exists $tree{$temp_id}) {
        push @parent_list, $tree{$temp_id}{name};
        $temp_id = $tree{$temp_id}{pid};
    }
    say join ",", @parent_list;
}

__DATA__
15,10,name3
10,#,name1
12,10,name2
5,12,name4
8,5,name5
[download]

which produces the following output:

$ perl test_ranks.pl
name3,name1
name1
name2,name1
name4,name2,name1
name5,name4,name2,name1
[download]

The solution you contemplated in your question is also relatively easy, except for the syntax used to sort the records according to the ranks, which may be regarded as a bit complicated for a Perl beginner. Here, I have added a rank field into the records and then sort the id's in accordance to the rank:

use strict;
use warnings;
use feature 'say';

my (%tree);
while (<DATA>) {
    chomp;
    my ($id, $pid, $name) = split /,/;
    $tree{$id} = {pid => $pid, name => $name, rank => $.};
}
for my $id (sort { $tree{$a}{rank} <=> $tree{$b}{rank} } keys %tree) {
    my @parent_list;
    my $temp_id = $id;
    while (exists $tree{$temp_id}) {
        push @parent_list, $tree{$temp_id}{name};
        $temp_id = $tree{$temp_id}{pid};
    }
    say join ",", @parent_list;
}

__DATA__
15,10,name3
10,#,name1
12,10,name2
5,12,name4
8,5,name5
[download]

which produces the same output as before.

Another possible way might be to read the input file again to get the id's in the proper order, but I wouldn't recommend that as that would be less efficient.

You may also take a look at the Hash::Ordered module on the CPAN, which implements a hash data structure with a builtin order.

Update: Fixed a typo: s/reparing the output/preparing the output/.