okay I'm there for the moment. Thank you all.
I finally put the regex, I was adviced to use, in the place where it works. Code below could be improved because when starting the first column with a 'space' or nothing there won't show up a 0 (zero) in the column but that also wasn't really where I was after.
Thangs again, rethinking the issue made me learn and get to a solution.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Table;

my $t =
  new Data::Table( [ [ "1", "2", "3" ], [ "10", "20", "2,3" ] ], [ "A"
+, "B" ],
    1 );

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

# to change , into . in respective columns
# not needed for a correct calculation of col3 though
$t->colsMap( sub { $_->[0] =~ s/,/\./g } );
$t->colsMap( sub { $_->[1] =~ s/,/\./g } );

# Return input if input is an integer, otherwise zero
sub to_int_or_zero {
    my $value = shift;
    if (
        defined($value)

        # the regex that above hints pointed at.
        && $value =~ /^-?\d+\.?\d*$/
      )
    {
        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;
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