It works for me with HTML::Tidy v1.06 and an older libtidy "vers 1st December 2004" on CentOS 4.

It fails with the same error as you with HTML::Tidy v1.08 and libtidy "vers 1 September 2005" on Ubuntu 6.06.

I haven't been able to convince 1.08 to accept the doctype yet, so I guess the quickest workaround would be a downgrade :-(