To get the keys:

use FreezeThaw qw(cmpStr);

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

Shorter:

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

If you want the actual *entries* (which is what you asked for), it can be shorter:

#        1         2               3         4         5
#23456789012345678901234      56789012345678901234567890

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

#        6         7
#2345678901234567890123456

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

This, by the way, is where 1 and '1' differ, see also DBI: when 1 != '1'.

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