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As an lvalue keys allows you to increase the number 
of hash buckets allocated for the given hash. This can gain
you a measure of efficiency if you know the hash is going 
to get big.

use Benchmark;
timethese( 100, {with => q{
             my %hash;
...
              }
           },
         });

Benchmark: timing 100 iterations of with, without...
      with: 14 wallclock secs (13.73 usr +  0.04 sys = 13.77 CPU) @  7
+.26/s (n=100)
   without: 14 wallclock secs (13.86 usr +  0.00 sys = 13.86 CPU) @  7
+.22/s (n=100)

1/16384
2/16384
...
8/16384
9/16384
10/16384

4/8
5/8
...
13/16
13/16
etc...