I was getting the error calling install.cgi for the first time.
I ended up installing from the command line, which worked fine, but I was then getting the "ContentType: Text/HTML" HTTP response header printed at the top of each page. I then set PerlSendHeader to On for the cgi directory and that seemed to fix the response header issue.
I then went back to see if the repsonse header problem was causing the failure of install.cgi and install.cgi now seemed to work when run from the browser.
Long story short...
Setting PerlSendHeader On, as stated in your documentation, seemed to fix my install.cgi problem.
Does this sound right to you?