It's not clear to me what you want. Maybe it's something like 

my $C;

if (@$ar) {
   $C = 1;
   foreach my $row (@$arr) {
      my $sum = 0;
      foreach my $ele (@$row) {
         $sum += $ele;
      }
      $C *= $sum;
   }
}

[download]

By the way, you shoulnd'y use $a and $b as variable names to avoid conflicts with some special variables.

Update: Or maybe 

my $cols = @{$arr->[0]};

my $C;

foreach my $col_idx (0 .. $cols-1) {
   $C->[$col_idx] = 1;
   foreach my $row (@$arr) {
      $C->[$col_idx] *= $row->[$col_idx];
   }
}

[download]

In reply to Re: Obtaining terms in an expansion by ikegami
in thread Obtaining terms in an expansion by randyk

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