use warnings;
  use strict;

  my $f1 = '1.txt'; # file that needs the treatments added on
  my $f2 = '2.txt'; # file that has all of the treatments and an underscore 
  my $f3 = '3.txt'; # output file

  my %ids;
  my $fh;

  open $fh, '<', $f2 or die;
  while (<$fh>) {
      chomp;
      my ($name, $id) = split /_/;
      $ids{$name} = $id;
  }
  close $fh;

  open my $fho, '>', $f3 or die;
  open $fh, '<', $f1 or die;

  while (<$fh>) {
      chomp;
      my @names = split /,/;
      foreach my $name (@names) {
          $name = $name . '_' . $ids{$name};
      }
      my $line = join(',', @names);
      print $fho $line, "\n";
  }

  close $fh;
  close $fho;