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use Benchmark;

...
    }
);

Benchmark: timing 1000 iterations of grep_seen, keys_map...
 grep_seen: 13 wallclock secs (11.17 usr +  0.00 sys = 11.17 CPU) @ 89
+.52/s (n=1000)
  keys_map: 30 wallclock secs (29.28 usr +  0.00 sys = 29.28 CPU) @ 34
+.15/s (n=1000)