Do you mean "Given a value, how can I find the key?". If so the short answer is: "Probably you can't".

The longer answer is that keys are unique for any given hash but values are not. For example a hash may contain the following key => value pairs:

apple  => red
ball   => red
lemon  => yellow
lime   => green
grass  => green
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The keys are unique, but the values (colours) are not. You can not ask for the single green object because there is more than one.

If there were a one to one mapping (keys and values unique) then you could generate a second hash using the first hash's values as keys and the first hash's keys as values - you would then have a reverse lookup.

If however you are asking "How do I get a list of the keys", the answer is trivial: keys.