Packages

       Perl provides a mechanism for alternative namespaces to
       protect packages from stomping on each other's variables.
       In fact, there's really no such thing as a global variable
       in Perl.  The package statement declares the compilation
       unit as being in the given namespace.  The scope of the
       package declaration is from the declaration itself through
       the end of the enclosing block, "eval", or file, whichever
       comes first (the same scope as the my() and local() opera­
       tors).

       ...

       Perl Modules

       A module is just a set of related functions in a library
       file, i.e., a Perl package with the same name as the file.
       It is specifically designed to be reusable by other mod­
       ules or programs.  It may do this by providing a mechanism
       for exporting some of its symbols into the symbol table of
       any package using it, or it may function as a class defi­
       nition and make its semantics available implicitly through
       method calls on the class and its objects, without explic­
       itly exporting anything.  Or it can do a little of both.