#!/usr/bin/perl

use strict;
use warnings;

use Benchmark qw 'cmpthese';

our %hash = map {$_, $_} 1 .. 100_000;

cmpthese (-10, { 
    keys    => '$a = 0; for (keys %hash) {$a += $_}',
    while   => '$b = 0; while ($_ = each %hash) {$b += $_}',
});
__END__
        Rate  keys while
keys  5.23/s    --  -26%
while 7.05/s   35%    --

##</code><code>##

for my $k1 (keys %hash) {
    for my $k2 (keys %hash) {
    }
}

for my $k (keys %hash) {
    %hash = ();
    ....
}