According to S06:

    my $pi is constant = 3;

Is this a special form?  If yes, what does it desugar to?

    my $pi is constant := 3;
    my $pi is constant ::= 3;

If not a special form, should this work? 

    my $pi is constant;
    $pi = 3;

If yes, should this pass compilation?

    my $pi is constant;
    $pi = 3 if (0|1).pick;
    $pi = 4;

If not, should this work?

    my ($x is constant, $y is constant) = (1, 2);

Thanks,
/Autrijus/