but I have used the DEFAULT in %EXPORT_TAGS with the function addition...

To clarify the previous answers slightly: the tag :DEFAULT is special. The import function does not look at $EXPORT_TAGS{DEFAULT} for this tag, so it doesn't matter that you have set this element in %EXPORT_TAGS.

If you change DEFAULT to MYTAG (or anything other than DEFAULT) in your module and program, then your addition subroutine is imported into your main package.