$ perl
use strict;
use warnings;
use Data::Table;

my $t = new Data::Table([ [ 1, 2, 3 ], [ 10, 20, " " ] ], [ 'A', 'B' ], 1);

# create emtpy column
my @newCol = ();
$newCol[ $t->nofRow - 1 ] = undef;
$t->addCol(\@newCol, 'adding_up');

# Put the formula in the empty column
$t->colsMap(
  sub {
    $_->[2] =
      ((defined($_->[0]) && $_->[0] =~ m/^\d+$/) ? $_->[0] : 0) +
      ((defined($_->[1]) && $_->[1] =~ m/^\d+$/) ? $_->[1] : 0);
  }
);
print $t->csv;

__END__
A,B,adding_up
1,10,11
2,20,22
3, ,3