I like Perl, and I think that changing the defaults is a sane thing to do. Starting a new script with dozens of "use strict", "use v5.30", etc is less than ideal and it's easy to forget something and shoot yourself in the foot. Almost 30 years will pass between Perl 5 and 7, and going back to the old values is just a matter of adding a few lines at the beginning of your old script. It's not a mess like the whole "Python 2/3" ordeal that was deeply more complex and painful.
Heck Perl in the last 30 years has been even more stable than C. I remember when gcc and clang switched from a default of GNU89 to GNU99 and it broke several packages who didn't set or check for the right flags; nobody bitched about it and just went on to fix their Makefiles.
I am not a Perl person, but it does sound to me like a lot of this thread is is missing what's going on, yeah. They're not changing the language, they're just changing the runtime to use better, more modern defaults. I don't know if you would be able to put in "use unstrict" or something and get back the old behavior, but any actively-maintained code is probably using most of these newer defaults anyway.
Heck Perl in the last 30 years has been even more stable than C. I remember when gcc and clang switched from a default of GNU89 to GNU99 and it broke several packages who didn't set or check for the right flags; nobody bitched about it and just went on to fix their Makefiles.