! /usr/bin/perl -w

use strict;

my %h = (
    a => 1,
    b => undef,
    c => 1,
    d => 1,
    e => undef,
);

my @def;

$def[ defined $_ ? 1 : 0]++ for values %h;

print "undefined=$def[0], defined=$def[1]\n";
__END__
produces:
undefined=2, defined=3

##</code><code>##

#! /usr/bin/perl -w

use strict;
use Benchmark qw/cmpthese/;

my %h;
my $nr = shift || 10000000;
$h{$nr} = ((rand() > 0.2) ? 1 : undef) while $nr--;

sub by_for {
    my $h = shift;
    my @def;
    $def[ defined $_ ? 1 : 0]++ for values %$h;
    @def;
}

sub by_grep {
    my $h = shift;
    (
        scalar (grep !defined, values %$h),
        scalar (grep  defined, values %$h),
    );
}

printf "by_for:  undefined=%d, defined=%d\n", by_for( \%h );
printf "by_grep: undefined=%d, defined=%d\n", by_grep( \%h );

cmpthese( -5, {
        for  => ' by_for( \%h ) ',
        grep => ' by_grep( \%h ) ',
    }
);

##</code><code>##

by_for:  undefined=2000300, defined=7999700
by_grep: undefined=2000300, defined=7999700
Benchmark: running for, grep for at least 5 CPU seconds...
       for:  5 wallclock secs ( 5.27 usr +  0.00 sys =  5.27 CPU) @ 163521.04/s (n=862318)
      grep:  6 wallclock secs ( 5.05 usr +  0.00 sys =  5.05 CPU) @ 224222.12/s (n=1131621)
         Rate  for grep
for  163521/s   -- -27%
grep 224222/s  37%   --

##</code><code>##

cmpthese( -300, {
        byfor  => sub { by_for( \%h ) },
        bygrep => sub { by_grep( \%h ) },
    }
);

##</code><code>##

by_for:  undefined=2398233, defined=9601767
by_grep: undefined=2398233, defined=9601767
Benchmark: running byfor, bygrep for at least 300 CPU seconds...
     byfor: 311 wallclock secs (311.09 usr +  0.03 sys = 311.12 CPU) @  0.06/s (n=20)
    bygrep: 313 wallclock secs (312.95 usr +  0.04 sys = 312.99 CPU) @  0.04/s (n=13)
       s/iter bygrep  byfor
bygrep   24.1     --   -35%
byfor    15.6    55%     --

##</code><code>##

sub by_diff {
   my $h = shift;
   my $defined = grep defined, values %$h;
   (
      scalar keys(%$h) - $defined,
      $defined,
   );
}

printf "by_for:  undefined=%d, defined=%d\n", by_for( \%h );
printf "by_grep: undefined=%d, defined=%d\n", by_grep( \%h );
printf "by_diff: undefined=%d, defined=%d\n", by_diff( \%h );

cmpthese( -600, {
       byfor  => sub {  by_for( \%h ) },
       bygrep => sub { by_grep( \%h ) },
       bydiff => sub { by_diff( \%h ) },
   }
);

##</code><code>##

by_for:  undefined=2398740, defined=9601260
by_grep: undefined=2398740, defined=9601260
by_diff: undefined=2398740, defined=9601260
Benchmark: running bydiff, byfor, bygrep for at least 600 CPU seconds...
    bydiff: 606 wallclock secs (604.93 usr +  0.11 sys = 605.04 CPU) @  0.08/s (n=51)
     byfor: 625 wallclock secs (623.32 usr +  0.09 sys = 623.41 CPU) @  0.06/s (n=40)
    bygrep: 604 wallclock secs (602.22 usr +  0.07 sys = 602.29 CPU) @  0.04/s (n=25)
       s/iter bygrep  byfor bydiff
bygrep   24.1     --   -35%   -51%
byfor    15.6    55%     --   -24%
bydiff   11.9   103%    31%     --