#!/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