in reply to HTTP::Proxy traffic calculating
$message is a HTTP::Response object when your filter is a response filter, so you can access the request via $message->request(). This should let you compute your stats in the response filter.
Please note that HTTP::Proxy does currently behave well with large request bodies: it loads the whole request body in memory before processing and sending it to the next hop. This is more than often okay, except when you upload very laaaarge files.
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Seekers of Perl Wisdom