# store some stuff in the "database":
  $db->{ record_1 } = {
    greeting => 'hello',
    perl     => 'rules',
    etc      => [ 42, 99 ]
  };

  $db->{ some_other_record } = {
    greeting => 'Oi, gente',
    perl     => 'r0x0rs',
    etc      => [ 38, 45 ]
  };
  # and so on

  # now find keys of all records that "rules"
  my @keys_of_records_that_rule = grep {
    $db->{ $_ }{ perl } eq 'rules'
  } keys %$db;