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{
my %cache;
...
    cmp ( $cache{$b} ||= substr($b, -(19+20), 19) );
}
}

            Rate       st memoized    naive      grt
st       10281/s       --      -6%     -51%     -54%
memoized 10930/s       6%       --     -48%     -51%
naive    20942/s     104%      92%       --      -5%
grt      22143/s     115%     103%       6%       --