Strange. I am using Dancer 1.3202, the same as the version of the source you linked to, but wget doesn't show a Content-Length header as being present:
$ wget -S http://foo/bar.pdf
--2017-01-12 15:43:26-- http://foo/bar.pdf
Resolving foo
Connecting to foo:80... connected.
HTTP request sent, awaiting response...
HTTP/1.1 200 OK
Date: Thu, 12 Jan 2017 14:43:26 GMT
Server: Apache/2.4.10 (Debian)
Content-Type: application/pdf
Content-Disposition: inline; filename="bar.pdf"
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Transfer-Encoding: chunked
Length: unspecified [application/pdf]
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