Large Project
size of code
number of developers
Note these can be useful for small projects too.
"Most Open Source projects start out small... they might just stay that way if the code is a mess." - Eric Andreychek <eric@openthought.net>
Learn the perlstyle guide; expand from there.
Style Guide
not only variable name conventions or where to put the spaces
"General guidelines that will make your programs easier to read, understand, and maintain." -- perldoc perlstyle
Programming standards and conventions -- Code Complete, by Steve McConnel