What do you have it for?

If it is for yourself, and you have learned the second edition pretty well, then I would move on to another book. If it is for something you give people to get them started, I would buy the third edition. If it is for yourself and you have not read the second edition, decide how much it is worth it to you.

As for me, with books which are not intended to be used as references (Learning Perl is not), if I buy it and don't learn it pretty well I feel like I wasted my money. (I try not to waste my money...) And if I don't finish one edition, I probably wouldn't finish the next.