use v5; # start with p5 code

    if ( $^O eq 'MSWin32' ) {

          # more p5 code

    } elsif ( $^O eq 'Android' ) {

        use 6.0.0; # switch to p6 code
        say "On $*OS"; # $*OS is p6 equivalent to $^O 
        multi sub foo(*@args) { ... }

    }

    say "Back to Perl 5 on $^O"