Sigh. No one has done it with regexes yet, and that makes me sad. :)

#!/usr/bin/perl

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

use strict;
use warnings;

local $_ = join '', sort { $a =~ s/.*,//r cmp $b =~ s///r } <DATA>;

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

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