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.
##</code><code>##

use Benchmark;
timethese( 100, {with => q{
             my %hash;
             keys( %hash ) = keys( %hash ) = 10000;
             foreach $i (1 .. 10000) {
                $hash{"key$i"} = $i;
             }
           },
           without => q{
              my %hash;
              foreach $i (1 .. 10000) {
                 $hash{"key$i"} = $i;
              }
           },
         });
##</code><code>##

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)

##</code><code>##

1/16384
2/16384
3/16384
4/16384
5/16384
6/16384
7/16384
8/16384
9/16384
10/16384

##</code><code>##

4/8
5/8
6/8
7/8
7/8
7/8
7/8
7/8
12/16
13/16
13/16
13/16
etc...