All you need to do is make Astro::Observation the factory. It will return a class depending on what argument it receives. Here is a simple way to implement Astro::Observation:

package Astro::Observation;

use Astro::Observation::Simple;
use Astro::Observation::Complex;

sub new {
  my $self  = shift;
  my $class = shift;

  return Astro::Observation::Simple->new(@_)  if 'simple' eq $class;
  return Astro::Observation::Complex->new(@_) if 'complex' eq $class;
  return undef;
}
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my $obs = Astro::Observation->new("simple",'foo');
my $obc = Astro::Observation->new("complex",'bar');
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UPDATE: once again - just do what the wise and resourceful Kanji said.

UPDATE x 2: that's what i was trying to do! :D
Here is a tar ball should anyone want to try this out.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)