Thanks for your responses Monks. I think I have progressed a little. I changed the query to exclude CLOB values that were null. However I do get a different error:

DBD::Oracle::st fetchrow_array failed: ORA-01406: fetched column value was truncated (DBD: ORA-01406 error on field 2 of 2, ora_type 112) at dwf(url).pl line 2807.

Do I need to increase buffer size in Perl?? If so, how do I do that??