((defined($_->[0]) && $_->[0] =~ m/^\d+$/) ? $_->[0] : 0)
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Am I correct if I assume that here $_ is defined as 'one single digit or maybe a few digits (d+, no spaces and stuff like that in between the digits) and then if this is not the case it should be a 0 (zero)?

Almost. It means that $_->[0] is

• defined, and
• one or more digits 0-9

I would have made it a function to make it more readable:

$ 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');

# Return input if input is an integer, otherwise zero
sub to_int_or_zero {
  my $value = shift;
  if (defined($value) &&
      $value =~ m/^\d+$/) {
    return $value;
  } else {
    return 0;
  }
}

# Put the formula in the empty column
$t->colsMap(
  sub {
    $_->[2] =
      to_int_or_zero($_->[0]) +
      to_int_or_zero($_->[1]);
  }
);
print $t->csv;

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