The message means that perl asked the OS for more memory and got a refusal. That basically only happens if you ran out of both physical memory and swap space (assuming swapping is enabled). The point (memory consumed) at which this happens depends not just on the data size/memory used by your process, but also by the memory in use by other processes in the system. So if the other processes--system and user--are using more memory today than yesterday, then your process will hit the limits of your system earlier.

Update: Tilly pointed out that the above information could be misleading to those coming along later. This reply is aimed specifically at the OP who is using a 64-bit OS and therefore unlikely to being affect by any address space limitations. Anyone using a 32-bit OS may encounter an os-dependant, 2 or 4 GB limit for a given (perl) process even though his RAM + swap totals more than 2/4 GB.