I've heard the argument that it allows you to switch off features introduced in the latest version but shouldn't it be done the other way round, ie. specify if you want limitations, not to enable what is available by default?
Perl should work that way, but Perl hasn't worked that way in any version of 1 through 5, and so you have a choice to make. Make that change arbitrarily with Perl 5.16 next year and for everyone running code written for earlier versions to edit that code to run on Perl 5.16, or do it the current way and ease upgrading worries.
(Even so, it's still a good idea to specify the version number of the Perl language against which you've written any program, no matter what Perl 5's behavior regarding default behaviors.)