/usr/share/doc/libxml-parser-perl/examples/ctest.dtd is in libxml-parser-perl 2.41-1build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | <!ATTLIST ctest magic CDATA "xyzzy">
<!ATTLIST la a NMTOKENS #IMPLIED>
|