Building pango-1.24.2, I configure with --enable-gtk-doc. During 'make':

cd . && \
        gtkdoc-mkdb --module=pango --source-dir=../pango --output-format=xml --expand-content-files="" --main-sgml-file=pango-docs.sgml --sgml-mode --output-format=xml
100% symbol docs coverage (801 symbols documented, 0 symbols incomplete, 0 not documented)
See pango-undocumented.txt for a list of missing docs.
The doc coverage percentage doesn't include intro sections.
Use of uninitialized value in concatenation (.) or string at /sw/bin/gtkdoc-mkdb line 938.
Use of uninitialized value in concatenation (.) or string at /sw/bin/gtkdoc-mkdb line 938.
Use of uninitialized value in concatenation (.) or string at /sw/bin/gtkdoc-mkdb line 938.
Use of uninitialized value in concatenation (.) or string at /sw/bin/gtkdoc-mkdb line 938.

I don't see it for other packages I've built recently, so I assume there's a mistake in pango-docs.sgml that is not being handled gracefully by gtk-doc. Would be nicer to emit a warning indicating that there's a problem in the source (not sure what kind of problem leads to this gtkdoc-mkdb situation) rather than a perl diagostic. I have gtk-doc-1.11 installed via fink.