mysubclass->new({ myclass => { value = 23 }});

##</code><code>##

package myclass;
use Class::Std;
my %value :ATTR( :name<value> :default(1));

1;

##</code><code>##

package mysubclass;
use base qw(myclass);

sub START {
    my ($self, $ident, $arg_ref) = @_;
    $self->set_value(123) if !defined $arg_ref->{value};
}

1;