Blessed arrayrefs will save a bit of memory. If the data being stored is simple enough that it can be serialized into strings, blessed references to strings are are also possible. They are likely to be the most memory-efficient representation of objects, but they can be a pain if your objects aren't very simple. Inside-out objects are also pretty memory-efficient if you have lots of objects but only a few fields, because it's one hash per field instead of one hash per object.
Do check whether it's really the objects causing the issue though — it might be something like a memory leak caused by more references to the objects than expected. (Cyclical references; objects getting closed over by coderefs; global variables holding references to the objects.)