...
$sth->execute;
my $all = $sth->fetchall_arrayref;
my @max = (0) x 3;
foreach my $row (@$all) {
    for (my $i = 0; $i < @$row; $i++) {
        $max[$i] = length $$row[$i] if length $$row[$i] > $max[$i];
    }
}