BUT: isn't this flag already set?

Yes, it seems to be set for the building of Bloom::Faster.I think it also needs to be set (but apparently wasn't) when the bloom library (libbloom.so) was built. So ... the suggestion seems to be that you need to first rebuild the libbloom library with -fPIC.

Mind you, I'm only going by what I've read in that perldl mailing list thread - mainly http://mailman.jach.hawaii.edu/pipermail/perldl/2008-June/003642.html, where the poster is talking about re-building the pgplot C library, not about re-building the PGPLOT perl module. (But I have no personal experience to rely on, here.)

Cheers,
Rob