my $stmt = "SELECT MAX(CHAR_LENGTH(spalte1)), MAX(CHAR_LENGTH(spalte2)), MAX(CHAR_LENGTH(spalte3)) FROM tabelle";
my $sth = $dbh->prepare( $stmt ) or die $dbh->errstr;
$sth->execute or die $dbh->errstr;

my ($max_row1, $max_row2, $max_row3) = $sth->fetchrow_array;