/usr/share/doc/libxml-libxml-perl/examples/complex/complex.dtd is in libxml-libxml-perl 2.0116+dfsg-1+deb8u2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | <!ENTITY % f SYSTEM "dtd/f.dtd">
%f;
|