sub set_avg {
    my @cols = @_;
    sub { $_->[-1] = sum(@$_[@cols]) / @cols }
}

$t->addCol(undef, 'v1-3');
$t->colsMap(set_avg(1 .. 3));

$t->addCol(undef, 'v4,5');
$t->colsMap(set_avg(4, 5));