2q eviction algorithm

check out it's efficiency

git clone https://github.com/grinya007/2q.git
cd 2q
zcat ids.gz | ./test.pl

with max size of 2000 entries it does:

worked out 1000000 keys
        hit rate:       72.689 %
        memory:         2.828 Mb
        time:           2.767 s

Cache::LRU with everything else being equal does:

worked out 1000000 keys
        hit rate:       64.762 %
        memory:         3.547 Mb
        time:           2.998 s

Cache::Ref::CAR with everything else being equal does:

worked out 1000000 keys
        hit rate:       69.292 %
        memory:         3.465 Mb
        time:           12.961 s