This is that line in the Net/Packet/UDP.pm source:

   $totalLength += $frame->l7->getLength if $frame->l7;
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So it is expecting that $frame->17 be an object that has a getLength method... in your case $frame->17 is your $data, which is your make_sollicit(), so the question is, what exactly is make_sollicit()? Can you rewrite it so that it returns an object that supports getLength() ?