This question is discussed at length in the excellent
Perl Best Practices book in the third item of Chapter 9, "Argument Lists: Always unpack @_ first". Notice that this chapter is available free online as the book's
In this item, TheDamian argues that either version is acceptable, with the shift-based version preferred when one or more arguments needs to be sanity checked or documented with a trailing comment. The most important thing though, is to unpack them at the start of the subroutine and to avoid accessing them directly as $_, $_ etc.