use warnings;
use strict;
use Carp;
use Data::Dumper;
local $SIG{__WARN__}=sub {(my $str=join(" ",@_))=~s/[\n\r]//g;die("\t@{[(caller(1))[3]]}() => $str\n")};

my $foo = {
     aa => { O => 'zz',  C => 'yy'  }, #
     bb => { O => 'xx',  C => 'xx'  },
};

sub mysort {return sort {$a cmp $b} @_}

sub eg_{my$h=pop;grep{!(($h->{$_}{O}||$;)eq($h->{$_}{C}||$;))}%$h}

print Data::Dumper->Dump([$foo],[qw(foo_before)]),"\n";

eval { print "  Elusion golf: ".join (" ",mysort eg_($foo))."\n"; };
print $@ if $@;
print Data::Dumper->Dump([$foo],[qw(foo_after)]),"\n";
__END__
$foo_before = {
                'aa' => {
                          'O' => 'zz',
                          'C' => 'yy'
                        },
                'bb' => {
                          'O' => 'xx',
                          'C' => 'xx'
                        }
              };

  Elusion golf: aa

##</code><code>##

$foo_after = {
               'HASH(0x1acf040)' => {},
               'aa' => {
                         'O' => 'zz',
                         'C' => 'yy'
                       },
               'bb' => {
                         'O' => 'xx',
                         'C' => 'xx'
                       },
               'HASH(0x1acef44)' => {}
             };