Read further down in the LWP::Request::Common:

           If you set the $DYNAMIC_FILE_UPLOAD variable
           (exportable) to some TRUE value, then you get back a
           request object with a subroutine closure as the con-
           tent attribute.  This subroutine will read the content
           of any files on demand and return it in suitable
           chunks.  This allow you to upload arbitrary big files
           without using lots of memory.  You can even upload
           infinite files like /dev/audio if you wish.  Another
           difference is that there will be no Content-Length
           header defined for the request if you use this fea-
           ture.  Not all servers (or server applications) like
           this.
[download]

Still no reason to write the code yourself. Remember... LWP has had many many man-years of coding put into it. Don't reinvent the wheel.

-- Randal L. Schwartz, Perl hacker