Writing large programs requires more discipline than writing small programs, due to the difficulty of managing all of the details of your program simultaneously. Abstraction (finding and exploiting similarities and near-similarities) and encapsulation (grouping specific details together and accessing them where they belong) are essential to managing this complexity.

Functions help, but functions by themselves aren't sufficient for the largest programs. Object orientation is a popular technique for grouping functions together into classes of related behaviors.

Perl 5's default object system is minimal. It's very flexible--you can build almost any other object system you want on top of it--but it provides little assistance for the most common tasks.