use Benchmark qw[ cmpthese ];

$h{ $_ } = 0 for 1 .. 100;
cmpthese( -3, { 
	for   => q[ $h{ $_ }++ for keys %h; ], 
	while => q[ $h{ $_ }++ while $_ = each %h; ] 
});

         Rate while   for
while 11937/s    --  -23%
for   15544/s   30%    --

$h{ $_ } = 0 for 1 .. 1000;
cmpthese( -3, { 
	for   => q[ $h{ $_ }++ for keys %h; ], 
	while => q[ $h{ $_ }++ while $_ = each %h; ] 
});

        Rate while   for
while 1165/s    --  -26%
for   1581/s   36%    --

$h{ $_ } = 0 for 1 .. 10000;
cmpthese( -3, { 
	for   => q[ $h{ $_ }++ for keys %h; ], 
	while => q[ $h{ $_ }++ while $_ = each %h; ] } );

        Rate   for while
for   80.0/s    --   -8%
while 86.9/s    9%    --

$h{ $_ } = 0 for 1 .. 5000;
cmpthese( -3, { 
	for   => q[ $h{ $_ }++ for keys %h; ], 
	while => q[ $h{ $_ }++ while $_ = each %h; ] 
});

       Rate while   for
while 207/s    --  -15%
for   243/s   17%    --

$h{ $_ } = 0 for 1 .. 8000;
cmpthese( -3, { 
	for   => q[ $h{ $_ }++ for keys %h; ], 
	while => q[ $h{ $_ }++ while $_ = each %h; ] 
});

       Rate   for while
for   119/s    --   -0%
while 119/s    0%    --