I like to initialize (and create) my fields in the constructor (even if only to undef), so my subclass constructors would look like:

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

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;
}
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depending whether it's a hash- or array-based object.

Nothing requires this, however. I could rely on the fact that non-existant hash and array fields evaluate to undef, but I prefer the self-documenting properties of my method.