$ createlang plperlu some-database
-- [start: full error message] --
createlang: language installation failed: ERROR: could not load library
"/usr/local/lib/postgresql/plperl.so": dlopen
'/usr/local/lib/postgresql/plperl.so' failed. (Shared object "libperl.so"
not found, required by "plperl.so")
-- [end: full error message] --
You can simply execute:
# ln -s /usr/local/lib/perl5/5.8.8/mach/CORE/libperl.so /usr/lib/libperl.so