open(I1,"Inventory.csv") or die "file1: $!";
$_ = <I1>;   # read column headings

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

open(I2,"Matrix.csv") or die "file2: $!";
$_ = <I2>;   # read column headings

while (<I2>) {  # get data
    chomp;
    my ($sku,$siz,$clr,$upc) = split /,/;
    $data{$upc} .= join ',', '', $sku, $siz, $clr;

}

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

for (sort keys %data) {
     my @content = "$data{$_},$_";
    if (grep(/.*,.*,.*,.*$/, @content)){@column = grep(/.*,.*,.*,.*$/, @content)};
    #@column = grep(/.*,.*,.*,.*$/, @content);
    print $fh_out "@column\n";
}