PL/SQL normally treads the whole modification as a single transaction. If you want to have some part of the code as seperate transaction, you can use the functionallity of autonomous transactions. The statement PRAGMA AUTONOMOUS_TRANSACTION has to be part of the declarative section. Cause the transaction is seperate you have to seperate specify a COMMIT and ROLLBACK.
DECLARE PRAGMA AUTONOMOUS_TRANSACTION; BEGIN -- Code goes here END;
Check also the full SQL Guide with lots of useful examples.