Perl 5 / Perl 7 Interoperability

• Quoting from the README:

        Right now you could change the defaults by using
        use p5 in a file to avoid enabling v7.0 standards.

        ...

        The final name could change and we could prefer alternates like
        use v7 and use v5 or use compat::p5 and use compat::p7.
        Right now by using p5 and p7 this allows to avoid some
        technical details and a global replace could be performed later
        in the development cycle.