I imagine that, similar to Perl 6, a Perl 7 binary might be called perl7, with files being called .p7 or .pl7, .pm7, etc. Using this interpreter or this file extension would be the same as a use v7; (and turn on the corresponding feature bundle, etc.).
Changing the file extension because of
Perl 6 Raku doesn't make sense to me. We currently can run multiple versions of Perl 5 Perl ( :) ) on a single machine with existing methods so why not continue with this approach?
The Python community navigated the 2.x to 3.x transition without changing the extension. They ended up changing the extensions after version 3.5 for reasons I don't know or care about, but they didn't put the version name into the extension. There are a lot of simple Python 2.7 programs that will run in 3.3. If the argument is that the 5 has become part of the language name then let's change it back to just Perl.
Another example is the C language. There are dialects such as Cyclone that have different extensions but the .c extension is still used for the C language.