Well, I notice the new Google Checkout has now just plunked onto the scene, and their code samples page has a conspicuous absence of perl. The developer docs appear to be quite thorough, seems like it's just a bunch of http POSTs using a well structured XML schema. CPAN turns up missing the Google::Checkout module, so I may get to it, but the project I'd need it for is starting some months from now...

Off the top of my head, you'd want the full CRUD interface for the whole object model they defined, I think, a la

   my $cart = Google::Checkout::Cart;
   $cart->add_item( 
       name => 'Bill',
       description => 'a pal',
       price => 'priceless', quantity => 2
   );
   $cart->post( 
      merchant_auth_encoded =>         
        q[MTIzNDU2Nzg5MDpIc1lYRm9aZkhBcXlMY0NSWWVIOHFR]
   );
[download]

but personally, I'd (probably) never use that, instead opting for the magical "hash to google checkout" function (..Google::Checkout::from_hash(%args) ? )

maybe it's not too bad of an idea to implement that function as a wrapper to XML::Simple with the right config, and running the result against google's schema for verification? Then doing the post as per google 's spec.
Anyhow, sorry I've got no code, I'm probably not qualified to write such an "industrial strength" module yet (don't make mistakes with banking code!), but I'd like to help if any were interested in leading the way. Or at least hear about your progress. Thanks!

Edit: g0n - removed readmore tags