how to unpack a C struct with length array preceding data array

johnlumby has asked for the wisdom of the Perl Monks concerning the following question:

my perl app receives a buffer containing this format

  {   
   short lengths[2];
   char data[???];
  }
where there are two items in data,  of length
lengths[0] and lengths[1] respectively.
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I have tried and tried to unpack this into 4 perl variables in a single unpack() call and failed. I had to resort to two unpack()'s, the first to extract the lengths followed by something like

eval('($junk1 , $junk2, $data0, $data1) = unpack "ssA' . $data0_length
+ . "A" . $data1_length . '",$rcvd_buf;');
[download]

which works fine, but can some kind Monk tell me how to do this in one unpack.

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