my new {
   my ($class, ...) = @_;
   my $self = $class->SUPER::new(...);
   $self->{...} = ...;
   $self->{...} = ...;
   return $self;
}

##</code><code>##


use constant IDX_...  => Parent::NEXT_IDX() + 0;
use constant IDX_...  => Parent::NEXT_IDX() + 1;
use constant NEXT_IDX => Parent::NEXT_IDX() + 2;

my new {
   my ($class, ...) = @_;
   my $self = $class->SUPER::new(...);
   push(@$self,
      ...,  # IDX_...
      ...,  # IDX_...
   );
   return $self;
}