It's the other way around. PerlSendHeader should be *ON*. From the ModPerl documentation:

The PerlSendHeader On directive tells mod_perl to intercept anything that looks like a header line (such as Content-Type: text/plain) and automatically turn it into a correctly formatted HTTP/1.0 header, the same way it happens with CGI scripts running under mod_cgi. This allows you to keep your CGI scripts unmodified.