Replacing tasklist.exe with ps, your code worked with my machine and it shows while/each is faster and smaller.
1. About Speed
I noticed from your code, I was very unfair to while/each because I lack value assignment with foreach/keys test function. When I change Benchmark test function from 
    foreach my $k (keys %h){
    }

[download]
to 
    foreach my $k (keys %h){
        my $v=$h{$k};
    }

[download]
, benchmark shows while/each becomes faster 
case 10000
hash ready ... count=10000
Sat Nov 12 01:13:29 2011
----------------------------------------
           Rate test_each test_keys
test_each 108/s        --      -28%
test_keys 151/s       40%        --
----------------------------------------
Sat Nov 12 01:14:22 2011

case 100000
hash ready ... count=100000
Sat Nov 12 01:14:23 2011
----------------------------------------
            Rate test_keys test_each
test_keys 4.63/s        --      -23%
test_each 6.02/s       30%        --
----------------------------------------
Sat Nov 12 01:15:20 2011

case 1000000
hash ready ... count=1000000
Sat Nov 12 01:15:32 2011
----------------------------------------
          s/iter test_keys test_each
test_keys   2.71        --      -30%
test_each   1.89       43%        --
----------------------------------------
Sat Nov 12 01:16:35 2011

case 2000000
hash ready ... count=2000000
Sat Nov 12 01:19:32 2011
----------------------------------------
          s/iter test_keys test_each
test_keys   5.69        --      -31%
test_each   3.91       46%        --
----------------------------------------
Sat Nov 12 01:21:49 2011

[download]

2. About memory
With ps command, while/each loop shows very very small memory usage. Memory allocation of "keys" is significant. 
$./025-6.pl -M1=keys -M2=pairs -N=1e6
hash built, starting timer
Took 3.931463 and 39.000MB extra memory for 1000000 using keys/pairs m
+ethod
$./025-6.pl -M1=each -M2=pairs -N=1e6
hash built, starting timer
Took 2.434757 and 1.000MB extra memory for 1000000 using each/pairs me
+thod

[download]
It seems I miss something with GTop.

In reply to Re^10: Finding the size of a nested hash in a HoH by remiah
in thread Finding the size of a nested hash in a HoH by Jeri

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