I didn't understand Benchmark. It's pity of me. I should have passed function as reference and didn't need timethese(). With 2 million test and revised my poor code with your advice, the difference becomes smaller but they didn't reverse. 

use strict;
use warnings;
use Benchmark qw/cmpthese timethese/;

my %h;
$|=1;

sub with_keys{
    foreach my $k (keys %h){
        #no proc
    }
    return;
}
sub with_each{
    while( my($k,$v)=each %h){
        #no proc
    }
    return;
}

for my $max ( qw(10000 100000 1000000 2000000) ){
    print "\ncase $max\n";
    %h=map { $_ => 1 } (1 .. $max);
    print "hash ready ... count=" . (scalar keys %h) ."\n";
    print scalar gmtime, "\n";
    print "-" x 40 ."\n";
    cmpthese(
        -20, #20 cpu time
        {
            'test_keys'=> \&with_keys,
            'test_each'=> \&with_each,
        }
    );
    print "-" x 40 ."\n";
    print scalar gmtime, "\n";
}

__DATA__
case 10000
hash ready ... count=10000
Fri Nov 11 13:27:23 2011
----------------------------------------
           Rate test_each test_keys
test_each 109/s        --      -61%
test_keys 281/s      158%        --
----------------------------------------
Fri Nov 11 13:28:12 2011

case 100000
hash ready ... count=100000
Fri Nov 11 13:28:12 2011
----------------------------------------
            Rate test_each test_keys
test_each 6.10/s        --      -32%
test_keys 8.93/s       46%        --
----------------------------------------
Fri Nov 11 13:29:04 2011

case 1000000
hash ready ... count=1000000
Fri Nov 11 13:29:09 2011
----------------------------------------
          s/iter test_each test_keys
test_each   1.88        --      -24%
test_keys   1.42       32%        --
----------------------------------------
Fri Nov 11 13:30:05 2011

case 2000000
hash ready ... count=2000000
Fri Nov 11 13:31:05 2011
----------------------------------------
          s/iter test_each test_keys
test_each   3.89        --      -24%
test_keys   2.96       31%        --
----------------------------------------
Fri Nov 11 13:32:43 2011

[download]

Thanks for your reply.

In reply to Re^7: 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

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.