DECLARE
temp_blob BLOB;
temp_clob CLOB;
amount integer :=2000;
dest_offset number(3) :=1;
src_offset number(3) :=1;
default_lang_ctx integer := 0;
warning integer :=1;

BEGIN
...
dbms_lob.converttoclob(temp_clob,temp_blob,amount,dest_offset,src_offset,190,default_lang_ctx,warning);
...
END;

SQL> DECLARE
  2    temp_blob   BLOB;
  3    temp_clob   CLOB;
  4    dest_offset NUMBER  := 1;
  5    src_offset  NUMBER  := 1;
  6    amount      INTEGER := dbms_lob.lobmaxsize;
  7    blob_csid   NUMBER  := dbms_lob.default_csid;
  8    lang_ctx    INTEGER := dbms_lob.default_lang_ctx;
  9    warning     INTEGER;
 10  BEGIN
 11    DBMS_LOB.CREATETEMPORARY(lob_loc=>temp_blob, cache=>TRUE, dur=>dbms_lob.SESSION);
 12    DBMS_LOB.CREATETEMPORARY(lob_loc=>temp_clob, cache=>TRUE, dur=>dbms_lob.SESSION);
 13
 14    DBMS_LOB.CONVERTTOCLOB(temp_clob,temp_blob,amount,dest_offset,src_offset,blob_csid,lang_ctx,warning);
 15  END;
 16  /

PL/SQL procedure successfully completed.