You basically need to maintain a membership map as an index of values, the reverse look up referred to below: 
my $search = "red";

my %fruit = (
   'apple' => ['red','green'],
   'kiwi' => 'green',
   'banana' => 'yellow',
  );

my %idx_fruit = (
    'red'    => {'apple' => 1 },
    'green'  => {'apple' => 1, 'kiwi' => 1 },
    'yellow' => {'banana' => 1},
  );

[download]
Note: %idx_fruit is a hash, but contains members as hash references. This is to provide natural semantics for managing the membership sets and accessing values (e.g., delete, exists, etc). The value => 1 is meaningless, but might be useful for some additional meta data. You could even have them point to the reference of the member in %fruit for maximized fun.

In reply to Re: Find all keys from a value in a hash by perlfan
in thread Find all keys from a value in a hash by Anonymous Monk

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