Another way would be to use the conditional operator inside a loop

# field number for sizes SM MD LG XL
my @fno =(9,12,15,18);
while (<DATA>) {  ## file2
  my @data = split(',',$_);
  my $ky = $data[0];
  if (exists $stock{$ky}) {

    # check each size
    for my $sz (0..3){
      $data[$fno[$sz]] = ( $stock{$ky}[$sz] eq '0' ) ? '' : '0' ;
    }

  }
  print (join(',', @data));
}
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poj