return new BerkeleyDB::Btree
               -Filename => $dbname,
               ......
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That's wrong. The way calling a constructor from a subclass works in perl is to pass on the $class argument to the superclass's constructor, like this:

  return $class->SUPER::new( ... )
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Which should make sure the final object will be in the $class class (which will contain the actual requested class - IOW your class, or some subclass of that), instead of the BerkelyDB::Btree class (provided its constructor is coded correctly)