use strict;

## simulate stock data from above script
my %stock;
$stock{'128'} = [0,224,0,0];
$stock{'12'} = [0,0,15,0];
$stock{'8'} = [32,0,0,0];

while (<DATA>) {  ## file2
  my @data = split(',',$_);
  if (exists $stock{$data[0]}) {
    my $ary_ref = $stock{$data[0]};
    my ($sm,$md,$lg,$xl) = @$ary_ref;

    $data[9]  = $sm unless ($sm);
    $data[12] = $md unless ($md);
    $data[15] = $lg unless ($lg);
    $data[18] = $xl unless ($xl);
  }
  print "@data\n";
}

# I've assumed you want the 'price' element (??) left alone unless 'qty' is zero
# in which case set it to zero.
__DATA__
128,1,Download,0,,TEXT,2,Size,2,??,SM,3,??,MD,4,??,LG,5,??,XL
12,1,Download,0,,TEXT,2,Size,2,??,SM,3,??,MD,4,??,LG,5,??,XL
8,1,Download,0,,TEXT,2,Size,2,??,SM,3,??,MD,4,??,LG,5,??,XL