The perlfunc manpage says about the keys function:
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.

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I decided to try benchmarking it to see how much it did increase efficiency and ran this 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;
              }
           },
         });

[download]

My results from this show that using keys didn't acually increase efficiency at all... 
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)

[download]

I would think that since this would make it so the hash wouldn't constantly have to be expanded this would greatly increase efficiency. Printing out the number of buckets filled and total number of buckets shows that without allocating the keys beforehand, the hash is reallocated somehow when expanded whereas it isn't when you do allocate: 
1/16384
2/16384
3/16384
4/16384
5/16384
6/16384
7/16384
8/16384
9/16384
10/16384

[download]
vs.
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...

[download]
Am I doing something wrong in my benchmark? Or is there some magic going on behind the scenes of hashes that make expanding the hash not an issue? Thanks

In reply to Using keys to increase hash efficiency by Boogman

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