#!/usr/bin/perl

use strict;
use warnings;
use Benchmark 'cmpthese';

my %hash = (
  foo => 1,     bar => 1,      baz   => 1,
  one => undef, two =>  undef, three => undef,
);

my ($defined, $undef, $count, @def);

sub for_values { 
    defined($_) ? $defined++ : $undef++ for values %hash;
}

sub grep_values { 
    $defined = scalar (grep  defined, values %hash);
    $undef   = scalar (grep !defined, values %hash);
}

sub grep_subtract {
    $defined = scalar (grep defined, values %hash);
    $undef = (scalar keys %hash) - $defined;
}

sub for_array {
    $def[ defined $_ ? 1 : 0]++ for values %hash;
}

cmpthese ( -5,
    {
        for => \&for_values,
        grep => \&grep_values,
        grep_two => \&grep_subtract,
        for_array => \&for_array,
    }
)
__END__

##</code><code>##

I'll just post the summary output from cmpthese: (perl 5.6.1)

              Rate for_array      grep       for  grep_two
for_array  82736/s        --       -8%      -12%      -44%
grep       90290/s        9%        --       -4%      -39%
for        94074/s       14%        4%        --      -37%
grep_two  148846/s       80%       65%       58%        --