If his background is Java, I'd say a critical part of his development will be to learn that in Perl the rules, especially the rules of object oriented programming, are largely up to the programmer to legislate. He is likely used to relying on the compiler to restrict heavily what he can do, and mistakenly conflate this with what he ought to do. TheDamian's book Object Oriented Perl is a must have for someone in the position you describe.