Bytes::Random::Secure uses a closure to share the same lazily-instantiated CSPRNG across several functions while avoiding making it accessable as a package global -- for several reasons. This could be achieved through other means, such as a private accessor function with its own state variable holding onto the RNG instance. Then each of the functions that shares the same RNG would use the accessor function to obtain a reference to the RNG instance, but it would be a little less convenient to implement, and would impose a minimum Perl version requirement for no good reason, which is something I really worked hard to avoid.
This isn't a slam-dunk endorsement of closures over state variables, since a similar outcome could be acheived using state variables with a little more work. And backward compatibility probably isn't one of the "real world" reasons you are interested in taking into consideration. But in this particular case, the simplicity that closures lend, and the advantage of backward compatibility conspired to give this approach an edge over more modern techniques.