NC.pm6

multi trait_mod:<is>(Routine $r, :$native! where Str|Callable|List) is export(:DEFAULT, :traits) {
    check_routine_sanity($r);
    say "STR/List/Callable";
    say $native.WHAT;
    $r does Native[$r, $native === True ?? Str !! $native];
}

multi trait_mod:<is>(Routine $r, :$native!) is export(:DEFAULT, :traits) {
    say "HELLO";
    say $native.WHAT;
}

multi trait_mod:<is>(Routine $r, NC-Library-Handle :$native!) is export(:DEFAULT, :traits) {
    say "Get a libhandle";
    check_routine_sanity($r);
    $r does Native[$r, $native];
}

Piko.p6

use NativeCall;

# First arg is just a 'key' name, second is basename of the lib. You will need etheir :version or :no-version
# ENV argument is optionnal. the corresponding ENV variable will be used instead of the rest if defined.
# The value returned is a NC-Library-Handle object (it's a 'hidden' role)
my \GUMBO_LIB = register-native-library('Gumbo', 'gumbo', :version(v1), :ENV<PERL6_LIBGUMBO>);

sub foo is native { * };

sub bar is native('hello') { * };

sub piko is native(GUMBO_LIB) { * };

zoutput

skarsnik@testperl6:~/devel/rakudo$ ./install/bin/perl6 -I lib testnc.p6 
HELLO
(Bool)
STR/List/Callable
(Str)
===SORRY!=== Error while compiling /home/skarsnik/devel/rakudo/testnc.p6
Can't use unknown trait 'is native' in a sub declaration.
at /home/skarsnik/devel/rakudo/testnc.p6:12
    expecting any of:
        rw raw hidden-from-backtrace hidden-from-USAGE
        pure default DEPRECATED inlinable nodal
        prec equiv tighter looser assoc leading_docs trailing_docs