Devel::Size will tell you the size of the Perl variables.

I think your idea of storing the boolean values or integer values of an object together in a string is sensible. If the size gains make it worthwhile and you still want fast access, you can also stuff all your objects into one string/array, for example by using Tie::Array::PackedC, but modifying these values becomes ugly.

If you have a large part of your needed memory in a hash, you can always "just" tie that hash to DB_File or one of the other btree implementations. Maybe you also can lazy-load some data or unload some data when it's not needed, but as all of that needs modification of the program, I'd go for one of the tie solutions that allow your program to run slower but otherwise unchanged.