Constants as members of a class

[TS xx]

Hello dear perl6 users,

I was in the need of declaring a member variable as a constant integer. After 
many syntax tryouts I came to this one:

class MyClass {
    has int $.myConst;

    method new () {
        return self.bless();
    }

    submethod BUILD () {
        constant $!myConst = 1;
    }

    method showMyConst () {
        print $!myConst;
    }
}

But I'm getting the followinf error message: "Twigil-Variable constants not yet 
implemented. Sorry."

The only place in the docs where I have found any reference to constants is in 
here: https://doc.perl6.org/language/variables#The_%3F_Twigil
But it's not what I am looking for :/

So my questions are:
Is the syntax right and the thing isn't implemented yet?
Is the syntax (or the full concept) wrong?
Do I have and old interpreter (This is perl6 version 2015.02 built on MoarVM 
version 2015.02)?

Thanks to all.

Kind Regards,
Emiliano