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$data {$a}{$b}
$data {$a}{$b}{$c}
$data {$d}{$e}

$data{join "\0", $a, $b}
$data{join "\0", $a, $b, $c}
$data{join "\0", $d, $e}

for (keys %data) {
   # you could do something with $data{$_} directly,
...
   my @keys = split /\0/;
   # ...
}