Not much, I followed theinstructions provided with the R/S-Perl package - compiled R with --enable-R-shlib, installed RSPerl with --with-in-perl; all of this worked as advertized, no errors of any kind. At this point, the above test works on the machine sans mod_perl, but produces the above error on the one that has mod_perl installed.

I just don't know what perl is doing at this point, so I don't know where to look for a solution (and getting rid of mod_perl isn't exactly an option). Why would R.so have any interaction whatsoever with Apache/Request/Request.so?