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  foreach my $j ($col_start .. $#column) {

...
    $x_tot[$j] += $x[$n[$j]][$j];
    $n[$j]++;
  }

my @n=0;
my @x_tot=0;
  my @g_jack_av=0;
  my @g_jack_err=0;

my @n;
my @x_tot;
  my @g_jack_av;
  my @g_jack_err;