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.