First, I have to confess that I am not an expert in database system ( I can't tell whether we are talking about an object repository, because I do not know what it is ).
Concerning my software, however I can explain a bit more:

• I store serialized Perl objects in a DB_File database. These objects are stored in a filesystem like tree; that is objects may contain other objects, such as directories contain files.
• There is an API to access objects; I have been working hard with Psh and Apache+mod_perl, but any other Perl program can access the object filesystem ( I have written other scripts to access the object filesystem, such as permanent daemons and cron jobs ). The thing is that as an administrator, I prefer the Psh command line to fix broken objects, but users have only access through the HTTP server.
• A great part of the concepts I implement, are taken from the Unix world, I just transposed them to the Perl object world.
• I have also written a bunch of other modules ( not in the tar you can get from geocities ):
  • Objects whose attributes are indexed ( either in an SQL database or in a BTREE DB_File ).
  • Objects that may contain a large number of other objects with fast access to subobjects.
  • Versionned objects ( under development ).