Correction. Closures are a distinctly different concept than objects. And making effective use of closures takes a different set of design skills than laying out a good OO design. (Neither is better than the other. They just don't translate very well.)
A closure is distinctly simpler in behaviour than an object. They aren't a less capable building block since you can implement an object system with closures. But a single closure represents a far less rich set of behaviours than a single object, and closures offer fewer tools than objects do to channel and constrain your designs along natural paths. (Flexibility is definitely a somewhat mixed blessing.)
Therefore it is incorrect to claim that closures are one of Perl's OO systems.