You're also documenting to Perl what you expect.

Remember that much of the difficulty of parsing Perl 5 (and enhancing Perl 5) is that you can only guess as to which features of Perl 5 you expect in any given file without an explicit version declaration. Sure, feature enables keywords, but you also get different semantics in 5.12 and 5.14, for example, with the use of unicode_strings.

With that said, use 5.12.4; has disadvantages compared to use 5.012;.