There've been a number of times when valuable new warnings are introduced in perl development versions, but can't be put in a maintenance version because people rightly expect previously warnings-free code to stay that way when upgrading from, say, 5.8.1 to 5.8.3. (And other times when warnings were added in maintenance releases and caused people problems.)
But it would be awfully nice to be able to have new warnings when you want them. It would be easy enough to add a perl-version-added-at tag to new warnings, and have some way to specify what perl version you have tested your code at, like use warnings "version"=>5.8.3, and if everyone would keep to that standard, there would be no problems. But when running existing code, what do you assume the default version to be? And how would you specify that you want all warnings, regardless of what version they were introduced in?
Anyone have suggestions for a workable interface?