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use FreezeThaw qw(cmpStr);

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

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

#        1         2               3         4         5
#23456789012345678901234      56789012345678901234567890
...
#2345678901234567890123456

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