use strict;
use warnings;

my %data;
{
   open(my $fh_in, '<', 'file1.txt')
      or die("Can't open first input file: $!\n");

   <$fh_in>; # Skip header.

   while (<$fh_in>) {
      chomp;
      my ($upc, @fields) = split /,/;
      push(@{$data{$key}}, @fields);
   }
}

{
   open(my $fh_in, '<', 'file2.txt')
      or die("Can't open second input file: $!\n");

   <$fh_in>; # Skip header.

   while (<$fh_in>) {
      chomp;
      my @fields = split /,/;
      my $upc = pop(@fields);
      push(@{$data{$key}}, @fields);
   }
}

{
   open(my $fh_out, '>', 'file3.txt')
      or die("Can't open output file: $!\n");

   foreach (sort keys %data) {
      print $fh_out (join(',', @$_), "\n");
   }
}