in reply to
Meaning of Maintanability and Functionality in Coding Perl
Ditto everything graff said. The point about looking at old code is one of the best ways I have learnt, when I have had to go back and maintain code which I wrote and found I couldn't understand chunks of it I knew I had work to do. I am probably overly verbose in my coding now but I know it will be clear when I come back to it in 1 year or 3. A lecturer in one of the few comp sci classes I took said that "a program is a letter from one programmer to another that a computer just happens to be able to understand", I think that is a good way to approach all your programs, like it is a letter to another programmer who you know nothing about.
Other resources which you might find useful are dominus's Program Repair Shop and Red Flags I, II and III. I haven't had a chance to read it yet but judging by the sample chapter you will also want Perl Best Practices by TheDamian.
Do not seek to follow in the footsteps of the wise. Seek what they sought. -Basho