The Perlog system is a Prolog system whose clauses and terms can persist on secondary storege as objects in a database. The unit of commitment is the module, and all atoms indirectly reference their module name, unless they are declared to be global. The module acts as a source of protection and reference to all its clauses, and also provides checks on import/export and on privacy, especially in the case of Abstract Data Types. This paper describes the structure of the module name space and the reasons for the decisions, particularly the use of metacalls to access procedures in other modules, and procedures with multiple definitions in different modules. A method for resolving global name clashes between previously committed modules is described. A model of the interpreter is presented in order to define the semantics of procedure call and assert and consult in a fully modular system. An example is given of a version of assert using B-tree routines in the database for clause storage. The system has been implemented in C and Prolog and PS-Algol under UNIX† on a VAX.‡