DECLARE
        orig_str varchar2(4000);
        len number;
        p_pos number;
        start_pos number;
        end_pos number;
        tempstr varchar2(300);
        prnt_str varchar2(300);
BEGIN
        orig_str:='aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaa,aaaaaaa,aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaa';
        select length(orig_str) into len from dual;
        dbms_output.put_line('orig_str length:'||len);
        if len < 256 then
                dbms_output.put_line(prnt_str);
        else
 start_pos := 1;
                end_pos := 255;
                WHILE ( start_pos <= len) LOOP
                dbms_output.put_line('start_pos, end_pos :0:'||start_pos||','||end_pos);
                select substr(orig_str,start_pos,end_pos) into tempstr
                        from dual;
                dbms_output.put_line('tempstr:'||tempstr);
                select instr(tempstr,',',-1,1) into p_pos from dual;
                dbms_output.put_line('p_pos:'||p_pos);
                end_pos := start_pos + p_pos -1;
                dbms_output.put_line('start_pos, end_pos:1:'||start_pos||','||end_pos);
                select substr(orig_str,start_pos,end_pos) into prnt_str from dual;
                dbms_output.put_line('prnt_str:'||prnt_str);
                start_pos := end_pos + 1;
                end_pos := end_pos + 255;
                dbms_output.put_line('start_pos, end_pos :2:'||start_pos||','||end_pos);
                tempstr:='';
                END LOOP;

        end if;
END;
/

~~~~
output
~~~~
SQL> @sam.sql
orig_str length:866
start_pos, end_pos :0:1,255
tempstr:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa,aaaaa
p_pos:250
start_pos, end_pos:1:1,250
prnt_str:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaa,
start_pos, end_pos :2:251,505
start_pos, end_pos :0:251,505
DECLARE
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character string buffer too small
ORA-06512: at line 20