Thanks a lot charob! i like DRY :-)

here are suggestion i got which use the colnames:

Version1:

my @mean123=map( ($t->elm($_,'value1')+$t->elm($_,'value2')+$t->elm($_
+,'value3'))/3.0, 0..$t->nofRow()-1);

$t->addCol(\@mean123, 'Mean123');

print $t->csv;
[download]

Version2:

$t->addCol(undef, 'Mean45'); # add a new column

for (my $i=0; $i< $t->nofRow; $i++) {
    $r=$t->rowHashRef($i);
    $t->setElm($i, 'Mean45', ($r->{'value4'}+$r->{'value5'})/2.0);
}
print $t->csv;
[download]

all works super fine- thanks a lot for your help!