mod.pm6

use MONKEY-TYPING;

augment class IO::Path {
    method blah { say "..." } 
}

output

nickl@localhost:~/perl6/zef$ perl6 test.pl6
CompUnit.new(from => "Perl6", name => "mod", extension => "pm6", precomp-ext => "moarvm", path => q|/home/nickl/perl6/zef/mod.pm6|.IO(:SPEC(IO::Spec::Unix)), has-source => Bool::True, has-precomp => Bool::False, is-loaded => Bool::False)
-------
True
CompUnit.new(from => "Perl6", name => "mod", extension => "pm6", precomp-ext => "moarvm", path => q|/home/nickl/perl6/zef/mod.pm6|.IO(:SPEC(IO::Spec::Unix)), has-source => Bool::True, has-precomp => Bool::False, is-loaded => Bool::False)
----

test.pl6

use lib '.';
use mod;
class CompUnit2 is CompUnit {
    has $!path;
    has $!has-precomp;
    has $.precomp-path;
    has $.build-output is rw;

    method precomp($out, |c) {
        $!precomp-path = $out //= self.precomp-path // "{$!path}.{$*VM.precomp-ext}";
        nextwith($!precomp-path, |c)
    }
}

my $in  = "mod.pm6";
my $out = "mod.pm6.moarvm";

my $cu = CompUnit2.new($in.IO.absolute);
say $cu.perl;
say "-------";
say $cu.precomp($out);
say $cu.perl;
say "----";