Very interesting point. Upvoted. But, at the same time, the Anonymous Monk's proposal did fit perfectly the OP's requirement: "a perl script that will find all instances of 2013 on a website and change it to 2014".
Agree that the OP's specs were met by s/2013/2014/g; the posting was just a way of wondering out loud whether the OP had considered potential collateral damage.