It is perhaps easier to describe "unclean" code. For example:
- Code with lots of code smells.
- Lots of magic numbers.
- A 5000-line main program.
- A sub that reads and writes global variables.
- A sub that does not have a single purpose; for example, instead of a sin function and a tan function, someone defines a sin_and_tan function.
- Some of a sub's parameters are not used.
- A sub that is 1000 lines long, aka "Big-Arsed Function".
- Duplicated code.
- Code with gaping security holes.
- Code that leaks resources.
- Code that is not thread-safe or signal-safe.
Conversely, some clean Perl code attributes are:
- Good variable naming.
- Minimize variable scope.
- Prefer lexicals to globals.
- Good commenting.
- Consistent indentation and visually pleasing layout.
- Easy to understand.
- Simple, Clear, General.
- Easy to use module interfaces.
- Comprehensive test suite.
Finally, TheDamian's new book
Perl Best Practices
provides much sound advice on writing clean Perl code.