BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
PL/SQL Release 11.2.0.4.0 - Production
"CORE	11.2.0.4.0	Production"
TNS for Linux: Version 11.2.0.4.0 - Production
NLSRTL Version 11.2.0.4.0 - Production

create table TMP_CLOB
(
  cdata CLOB
)

SELECT LENGTH(cdata) FROM tmp_clob;

65410

DECLARE
  l_clob CLOB;
BEGIN
  SELECT cdata
    INTO l_clob
    FROM tmp_clob;
  l_clob := convert(l_clob, 'UTF8');
  dbms_output.put_line(LENGTH(l_clob)); 
END;

130820

DECLARE
  l_clob CLOB;
BEGIN
  SELECT cdata
    INTO l_clob
    FROM tmp_clob;
  l_clob := convert(src => l_clob, destcset => 'UTF8');
  dbms_output.put_line(length(l_clob));
END;

ORA-06502: PL/SQL:  ошибка числа или значения
ORA-06512: на  line 7