use warnings;
use strict;
my $foo = {
     aa => { O => 'zz',  C => 'yy'  }, #
     bb => { O => 'xx',  C => 'xx'  },
     cc => { O => undef, C => 'ww'  }, #
     dd => { O => 'vv',  C => undef }, #
     ee => { O => undef, C => undef },
     ff => {             C => undef },

     gg => { O => 0,     C => 0     },
     hh => { O => undef, C => 0     }, #
     ii => { O => undef             }, #
};

#Only does defined not exists
#98
sub d {$a=pop;grep{$b=$$a{$_};defined$$b{O}!=defined$$b{C}||defined$$b{O}&&$$b{O}ne$$b{C}}keys%$a}

#Does defined and exists
#126
sub f {$a=pop;grep{$b=$$a{$_};exists$$b{O}!=exists$$b{C}||defined$$b{O}!=defined$$b{C}||defined$$b{O}&&$$b{O}ne$$b{C}}keys%$a}

# [tadman]s original solution compressed
#162
sub t {my($h)=@_;grep{!exists($h->{$_}->{O})||(defined($h->{$_}->{O})?(defined($h->{$_}->{C})?($h->{$_}->{O} ne $h->{$_}->{C}):1):defined($h->{$_}->{C}))}keys%$h}


print "d():\n",join("\n",sort {$a cmp $b}d($foo)),"\n\n";
print "f():\n",join("\n",sort {$a cmp $b}f($foo)),"\n\n";
print "t():\n",join("\n",sort {$a cmp $b}t($foo)),"\n\n";

__END__
d():
aa
cc
dd
hh

f():
aa
cc
dd
ff
hh
ii

t():
aa
cc
dd
ff
hh