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use strict;
use warnings;
...
   Hash    => sub { hash_lookup()    },
   HashRef => sub { hashref_lookup() },
};

Benchmark: timing 50000 iterations of Fn, Hash, HashRef...
       Fn: 28 wallclock secs (27.56 usr +  0.00 sys = 27.56 CPU) @ 181
+4.03/s
     Hash: 11 wallclock secs (11.62 usr +  0.00 sys = 11.62 CPU) @ 430
+1.08/s
  HashRef: 12 wallclock secs (12.02 usr +  0.00 sys = 12.02 CPU) @ 416
+1.12/s