$h{ $_ } = $_ for 0 .. 1e6;;

cmpthese 1,{
    a=>q[
        while( ( $k, $v ) = each %h ){
            my $x = "$k:$v";
        }
    ],
    b=>q[
        for my $k ( keys %h ){
            my $x = "$k:$h{$k}";
        }
    ],
};;

  s/iter    b    a
b   8.67   -- -83%
a   1.52 472%   --