($x eq $y)

##</code><code>##

((defined($x) && defined($y) && ($x eq $y))
     || (!defined($x) && !defined($y))

##</code><code>##

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     },
};

##</code><code>##

sub f
{
        my ($h) = @_;
        grep
        {
                !exists($h->{$_}->{O})
                || (defined($h->{$_}->{O})?
                        (defined($h->{$_}->{C})?
                                ($h->{$_}->{O} ne $h->{$_}->{C})
                                : 1)
                        : defined($h->{$_}->{C}))
        } keys %$h
}