Having use 5.whatever imply use compat::perl5 would be a start, at least. But it wouldn't be a 100% solution because a lot of code doesn't bother to specify a Perl version.

IMO, the most straightforward solution seems to be to add an empty compat::perl5 module to the core Perl 5 distribution (and on CPAN), so that Perl 5 will recognize use compat::perl5 as a valid statement, but one which doesn't actually do anything.