IIRC perl 5.8 changed the hash function... It may be that it's performing differently for your key set in such a way that more buckets are needed to store the values (arguably a good thing), to the point where Perl cannot allocate a contiguous chunk of memory for the bucket array (it probably needs to do that but I'm not sure).

Another problem could be that the new machine is shipping with soft/hard ulimits set differently by default. Check ulimit on the command line before running. Tweaking the hard limits may require a kernel recompilation.

Good luck!