#!/usr/bin/perl
use Benchmark qw(:all :hireswallclock);
$count = 1000;
for(1..$count){
	$hash{$_} = $_;
}
timethese($count, {
	'each' => sub {
		while (my ($key, $value) = each %hash) {
			$hash{$key} = 1; 
		}
	},
	'foreach keys' => sub {
		foreach (keys %hash) {
			$hash{$_} = 1; 
		}
	},
	'for keys' => sub {
		for (keys %hash) {
			$hash{$_} = 1; 
		}
	},
} );

Benchmark: timing 1000 iterations of each, for keys, foreach keys...
      each: 3.46613 wallclock secs ( 3.40 usr +  0.01 sys =  3.41 CPU) @ 292.83/s (n=1000)
  for keys: 1.98373 wallclock secs ( 1.97 usr +  0.00 sys =  1.97 CPU) @ 507.10/s (n=1000)
foreach keys: 2.02901 wallclock secs ( 1.98 usr +  0.00 sys =  1.98 CPU) @ 504.54/s (n=1000)