# Pastebin sky3GWnM
OLX-RJ233:Injector fernando$ cat test-inject-param.p6 
multi trait_mod:<is>(Parameter:D $p, :$injected!) {
    $p.set_default({"12345"});
}
multi trait_mod:<is>(Routine:D $r, :$changed!) {
    $r.signature does role {
        method decrement-arity(Int $num) {
            my $sig = $r.signature;
            my $arity = $sig.^attributes.first(｢$!arity｣);
            $arity.set_value($sig, $arity.get_value($sig) - $num)
        }
    }
    $r.signature.decrement-arity: 1
}
sub bla($a is injected) is changed {
    $a.say
}
bla
OLX-RJ233:Injector fernando$ perl6 test-inject-param.p6 
===SORRY!=== Error while compiling test-inject-param.p6
Calling bla() will never work with declared signature ($a = -> ;; $_? is raw { #`(Block|140404727786232) ... })
at test-inject-param.p6:17
------> <BOL>⏏bla
OLX-RJ233:Injector fernando$