perldoc HTTP::Response says:

       $r->base
           Returns the base URI for this response.  The return
           value will be a reference to a URI object.

           The base URI is obtained from one the following
           sources (in priority order):

           1.  Embedded in the document content, for instance
               <BASE HREF="..."> in HTML documents.

           2.  A "Content-Base:" or a "Content-Location:" header
               in the response.

               For backwards compatability with older HTTP imple-
               mentations we will also look for the "Base:"
               header.

           3.  The URI used to request this response. This might
               not be the original URI that was passed to
               $ua->request() method, because we might have
               received some redirect responses first.

           When the LWP protocol modules produce the
           HTTP::Response object, then any base URI embedded in
           the document (step 1) will already have initialized
           the "Content-Base:" header. This means that this
           method only performs the last 2 steps (the content is
           not always available either).
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-- Randal L. Schwartz, Perl hacker