why do I have to type use 5.016; at the top of every script?
You don't. You only need to this if you want version 5.16 of the language. Having the features always on or on by default would break programs, which is the very reason they were made features you need to turn on.
If we've admonished for so many years to (almost) always use "my" variables for proper scoping surely it is now time to makey "my" implicit?
Because having you declare them finds spelling errors. This is the worse suggestion for a change in Perl I have ever heard.
(I'm assuming you're not suggesting that we'd no longer be able to declare them explicitly. That would be even worse.)