Again, if you have to remember to do a particular thing to get benefits, it hurts the Perl community. For example, use strict; use warnings; should have been the default a long time ago, with additional action taken to disable them.
So, Joe SysAdmin does an upgrade one day, and finds half of the
old perl scripts on the system are broken. This is not something
you want to have happen, not when we're still fighting the
fallout from the "perl is dead" smear campaign.
I could see an argument that stuff like "use 5.10.0" should've
implied strict and warnings also... or for that matter that
cromatic's Modern::Perl should ship with the core library, it's a
lot easier to type correctly (I had to double-check that you
really need "5.10.0").
Backwards compatibility is really and truly important, and it
remains important-- it's one of the things that perl has always
gotten right (and the new kids keep screwing up).