url
  http://example.com/callback.txt
headers
  Content-Type: text/plain
  Content-Length: 42
content_cb
  my @content = qq{abcdefghijklmnopqrstuvwxyz1234567890abcdef};
  sub { shift @content }
----------
PUT /callback.txt HTTP/1.1
Host: example.com
Connection: close
User-Agent: HTTP-Tiny/VERSION
Content-Type: text/plain
Content-Length: 42

abcdefghijklmnopqrstuvwxyz1234567890abcdef

----------
HTTP/1.1 201 Created
Date: Thu, 03 Feb 1994 00:00:00 GMT
Location: http://example.com/callback.txt
Content-Length: 0