If you had a stale Apache header layout in one of the include paths during the build process, you will see the message "mod_perl.c is incompatible with this version of Apache" when you try to execute httpd. Find the file ap_mmn.h using find, locate, or another utility. Delete this file and rebuild Apache. The Red Hat Linux distribution usually installs it in /usr/local/include.
Before installing mod_perl-enabled Apache from scratch, it's a good idea to remove all the pre-installed Apache modules, and thus save the trouble of looking for files that mess up the build process. For example, to remove the precompiled Apache installed as a Red Hat Package Manager (RPM) package, as root you should do:
panic# rpm -e apache
There may be other RPM packages that depend on the Apache RPM package. You will be notified about any other dependent packages, and you can decide whether to delete them, too. You can always supply the —nodeps option to tell the RPM manager to ignore the dependencies.
apt users would do this instead:
panic# apt-get remove apache