Two ideas come to my mind:

1. A bug in Perl
2. Someone is replacing Exporter::Heavy::heavy_export() out from under you

but it sounds like you added debugging print statements directly to heavy_export() which would rule out #2. I'd probably install a different version of Perl (keeping your current version untouched) and see if the problem persists with that version.

        - tye (but my friends call me "Tye")