I think you'll find you've at least got an error here:

if(!$hash{$c_id}{$f_var}) {
     $hash{$c_id}{$f_var} = $d_id; 
}
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as I think a) that will always test true (I think you want to look at perldoc -f exists) and b) you are only going to get the first c_id/f_id and you didn't say that is what you wanted - you said remove duplicates.