/usr/share/doc/libxml-libxml-perl/examples/complex/dtd/f.dtd is in libxml-libxml-perl 1.89+dfsg-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | <!ENTITY % g SYSTEM "g.dtd">
<!ELEMENT doc ANY>
%g;
|