I'm only starting out on this stuff, but it sounds to me like you're looking for a Hash of Hashes where the key of the first level of hashes is the filetype, the key of the second level would be the filename and the value would be the modification time.

I'm not sure if it's possible but I'd guess it'd be initialised something like:

%filedata{substr($filename, 0,2)} = {
     %{$filename} = $mod_time
   };
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Updated: I checked How do I make a hash of hashes? and it looks like the initialization should be something like:

%filedate{substr($filename, 0, 2)} = {
     $filename => $mod_time
   };
[download]

But today you took me walking, Through a land that we have lost,
While our children sit at websites, With no access to the cost