use FreezeThaw qw(cmpStr);

sub f {
    my ($h) = @_;
    return grep cmpStr(@{$h->{$_}}{qw/C O/}), keys %$h;
}

##</code><code>##

use FreezeThaw 'cmpStr';sub f{my($h)=@_;grep{!exists$$h{$_}{O}||cmpStr@{$$h{$_}}{C=>'O'}}keys%$h}

##</code><code>##

#        1         2               3         4         5
#23456789012345678901234      56789012345678901234567890

use FreezeThaw 'cmpStr';sub f{grep{!exists$$_{O}||cmpStr

#        6         7
#2345678901234567890123456

@$_{C=>'O'}}values%{$_[0]}}