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)