In regards to the use of __CONSTANT__, these are simply substition values. From the POD:

To assist with writing SQL that is inheritable into subclasses, severa
+l additional substitutions are available here: __TABLE__, __ESSENTIAL
+__ and __IDENTIFIER__.  These represent the table name associated wit
+h the class, its essential columns, and the primary key of the curren
+t object, in the case of an instance method on it.
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William