Note that the default error_mode sends output to the browser:

   error_mode
       Indicates how errors are returned to the caller.  The choices are fatal, meaning die
       with the error, and output, meaning output the error just like regular output.

       The default under Apache and CGI is output, causing the error to be displayed in the
       browser.  The default for standalone mode is fatal.

(From perldoc HTML::Mason::Request.)

I have a dread of mod_perl under RedHat, but I haven't used it for several years so I don't know what the current status is. What you're describing sounds like something that might happen there (requests mysteriously dropped as something segfaults), though, which is why I asked.

I'd suspect your code first, though...