$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;