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  foreach my $j ($col_start .. $#column) {
    my $nIndex = $n[$j];
...
    $x_tot[$j] += $x[$nIndex][$j];
    $n[$j]++;
  }

  foreach my $j ($col_start .. $#column) {
    my $nIndex = $n[$j] // 0;
...
    $x_tot[$j] += $x[$nIndex][$j];
    $n[$j]++;
  }