“A database transaction” is an all-or-nothing boundary:   either everything happens, or nothing does.   Depending on the system, transactions may or may not be nested.   Bulk operations usually depend on intelligent use of transactions to avoid time-consuming “physical commits.”

Yes, this sounds like a homework question.   (Or, a question posted by a far-away offshore contractor who quoted the lowest price?)   Short answer is ... “Google it.”   And, pray.   Plenty of stuff has already been written on this subject, which really has nothing specifically to do with Perl or any other language.