$_ = <I1>;   # read column headings
my @columns= split /,/;

##</code><code>##

use strict;
my %data;

open(I1,"file1") or die "file1: $!"
$_ = <I1>;   # read column headings
my @columns= split /,/;

my %line;
while (<I1>) {  # get data
    chomp;
    @line{@column} = split /,/;
    $data{$line{'upc'}} = $line{'qty'};
}

open(I2,"file2") or die "file2: $!"
$_ = <I2>;   # read column headings
my @columns= split /,/;

while (<I2>) {  # get data
    chomp;
    @line{@column} = split /,/;
    $data{$line{'upc'}} .= join ',',@line{@columns},'';

# or: $data{$line{'upc'}} = join ',', $data{$line{'upc'}},@line{@columns};
}

for (sort keys %data) {
    print $data{$_}\n";
}