For any future searchers ..

I saw Damian yesterday and chatted to him about it. The way to do it is for the subclass to override new:

sub new {
    my ($class, $args) = @_;
    $args->{value} = 23;
    return $class->SUPER::new($args);
}
[download]

new() is actually installed into the parent class by Class::Std (rather than inherited), and because its receiving the caller, its able to do the right thing.

This is a workaround though. The next release of Class-Std will have a PREBUILD() method that runs before BUILD(). It goes in the opposite direction ("up" the inheritance tree) and allows the arguments to be changed before BUILD() gets them. So this will simply become:

sub PREBUILD {
    my ($class, $args) = @_;
    return { %{$args}, value => 23 };
}
[download]