From the look of it (source of 5.005/Exporter.pm) there is an undocumented variable %EXPORT which keys are like @EXPORT_OK, only they override @EXPORT_OK and @EXPORT in that they define which could optionally exported. The effect is if a symbol is in @EXPORT but not in %EXPORT you can it only forcibly export. ':DEFAULT' should result in: "$sym" is not exported by the $pkg module, for $sym=that symbol.

I hope I made myself understood.