%:: is a short name for %::main. The general name of the symbol table of any package is %::package
See the "Symbol Tables" section of perlmod for some usages.
See Dominus's "Tricks of the Wizards" class for more interesting uses, like how to do lightweight exporting without using the Export module.

And yes, Exporter is written in Perl, in two pieces:

  Exporter.pm
  Exporter/Heavy.pm

Update: added text below
Finally, here are some enlightening discussions on some advanced uses:

• Re: Re: Defining a subroutine in another package
• Adding autoloaded methods to symbol table with using strict refs
• overriding import; good or bad?
• Are custom imports a good idea?

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