From RFC 2616, HTTP/1.1, section 14.13: 

   The Content-Length entity-header field indicates the size of the
   entity-body, in decimal number of OCTETs, sent to the recipient or,
   in the case of the HEAD method, the size of the entity-body that
   would have been sent had the request been a GET. [...]

   Applications SHOULD use this field to indicate the transfer-length 
+of
   the message-body, unless this is prohibited by the rules in section
   4.4. [...]

   Note that the meaning of this field is significantly different from
   the corresponding definition in MIME, where it is an optional field
   used within the "message/external-body" content-type. In HTTP, it
   SHOULD be sent whenever the message's length can be determined prio
+r
   to being transferred, unless this is prohibited by the rules in
   section 4.4.

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Check with your LWP docs to see how to access the header.

In reply to Re: Determining file size over HTTP connection by gaal
in thread Determining file size over HTTP connection by mellin

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