Object Oriented Programming: logical modules are sometimes better OO classes ...

As for whether and when to use OO, my simple rule of thumb is to ask "do I need more than one?": if the answer is yes, an object is indicated; if the answer is no, a module.

A (non Perl-specific) design checklist (derived from On Coding Standards and Code Reviews):

• Coupling and Cohesion. Systems should be designed as a set of cohesive modules as loosely coupled as is reasonably feasible.
• Testability. Systems should be designed so that components can be easily tested in isolation.
• Data hiding. Minimize the exposure of implementation details. Minimize global data.
• Interfaces matter. Once an interface becomes widely used, changing it becomes practically impossible (just about anything else can be fixed in a later release).
• Design the module's interface first.
• Design interfaces that are: consistent; easy to use correctly; hard to use incorrectly; easy to read, maintain and extend; clearly documented; appropriate to your audience. Be sufficient, not complete; it is easier to add a new feature than to remove a mis-feature.
• Use descriptive, explanatory, consistent and regular names.
• Correctness, simplicity and clarity come first. Avoid unnecessary cleverness. If you must rely on cleverness, encapsulate and comment it.
• DRY (Don't repeat yourself).
• Establish a rational error handling policy and follow it strictly.