#! perl -w
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     }, #
};

#2345678901234567890123456789012345678901234567890123456789012345678901234567890123
sub f{no warnings;$a=pop;grep{!exists$a->{$_}{O}or$a->{$_}{O}ne$a->{$_}{C}}keys%$a} # 83!

print "@{[f($foo)]}\n";
__DATA__
C:\test>201598
dd ff aa hh cc
C:\test>