There's actually two directories that need to be added to @INC, the one that allows it to locate the .pm, and the one that allows it to locate the .so.
Not in my experience.
If you add a single location to @INC that will locate the
.pm file, then that's all you need to do - assuming that you've located the
.so in the location (relative to the
.pm file) where perl will look for it.
Of course, if you've placed the
.so file in some other location (where perl won't look for it) then you
will need to add a second path to @INC that will enable the
.so file to be found by perl.
I guess
you are referring to the latter scenario.
Cheers,
Rob