It is perhaps easier to describe "unclean" code. For example:
- Code with lots of code smells (see also wikipedia code smell)
- 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.
Update: See also Is it correct? by GrandFather:
- Does it work correctly?
- Do you understand it?
- Would anyone else understand it?
- Will you understand it in a month's time?
- Does it strike a good balance between terseness and verbosity?
- Could you make changes to it without it being likely to break in unforeseen ways?
- Is it fast enough?
Update: See also: On Coding Standards and Code Reviews