5.10 introduced new syntactic constructs such as say. If older code (that was written with an older version) already had a function called say(), Perl 5.10 would error out because of a conflict. That is why the new functions and constructs are only available through a pragma.
You are supposed to declare the minimum version of Perl that your code demands. This would include regular expression features and whatnot. It is safe to say use 5.014 but if someone will try to run your code with 5.12, he will get an error whether or not you actually use 5.14's new features. This is largely just a convenience for a person trying to figure out whether his version of Perl is good enough for your script.