I assigned the LongReadLen value like below and also i got the exact length, eventhough i am getting the same error.

my $new_sql = <<EOS;
        select <col_name> from <table_name>
EOS

$dbh->{LongReadLen} = $dbh->selectrow_array(qq{
      SELECT dbms_lob.getlength(<col_name>)
      FROM <table_name>
 });
print "CLOB Length: ".$dbh->{LongReadLen};
my $sth = $dbh->prepare($new_sql);
$sth->execute();
my @rows;
while(my $cols = $sth->fetchrow_hashref()) {
        push(@rows, $cols);
}
$sth->finish();
[download]

Anything wrong in that?