Super Search is your friend. The question you are asking comes up quite frequently here at the Monastery. This search turned up the following (amongst many other hits):
Use it, and with luck, your search might be just about over.
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5.003 (Camel Book 2nd Edition)
5.6 (Camel Book 3rd Edition)
5.8 (stable version for >5 years)
5.10 (many new features now in common use)
5.16 (Camel Book 4th Edition)
5.18 (hash overhaul)
5.26 (no "." in @INC)
5.36 (subroutine signatures)
Dynamic (no standard; code dictates VERSION)
Other (discuss in comments)
Results (32 votes). Check out past polls.