Other monks have pointed you at the feature documentation, but I'd like to expand on Anonymous monk's suggestion that you leave use strict; in your scripts even if it is automatically enabled by use 5.012;.
If you rely on use VERSION; and someone comes along later and modifies the code to run on an older perl version, they will remove or modify the use VERSION which may change the pragmas you are running under, and not all of them will immediately cause errors (losing say would be immediately obvious, but losing strict or unicode_eval would be more subtle).
Since strict is always relevant, I still include use strict; regardless of version. I consider a comment sufficient for other non-keyword features that I actuall make use of in the code. For example:
use 5.016; # for unicode_eval and lvalue substr