Even being able to successfully start Apache does not necessarily mean that its C sources are installed. Most distros only install the Apache runtime environment, i.e. compiled program, etc. (when you install the apache package), but not the source/header files. You typically need to install them separately via "*-dev" or "*-devel" packages.

If you intend to use packages supplied by the disto (or are generally not experienced with compiling stuff yourself), it's usually easier to also use the precompiled mod_perl distro package, if one is available...

@Khen1950fx - Thanks, that dir you list is exactly where mine came up after running the 'whereis' cmd. @almut - I did some digging, and here's what I got:

#rpm -qil httpd-2.2.3-11.el5
Name        : httpd                        Relocations: (not relocatab
+le)
Version     : 2.2.3                             Vendor: Red Hat, Inc.
Release     : 11.el5                        Build Date: Mon 06 Aug 200
+7 11:24:29 AM UTC
Install Date: Wed 13 May 2009 11:23:46 PM UTC      Build Host: ls20-bc
+2-14.build.redhat.com
Group       : System Environment/Daemons    Source RPM: httpd-2.2.3-11
+.el5.src.rpm
Size        : 3054126                          License: Apache Softwar
+e License
Signature   : DSA/SHA1, Mon 27 Aug 2007 01:38:50 PM UTC, Key ID 532681
+0137017186
Packager    : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>
URL         : http://httpd.apache.org/
Summary     : Apache HTTP Server
Description :
The Apache HTTP Server is a powerful, efficient, and extensible
web server.
/etc/httpd
/etc/httpd/conf
/etc/httpd/conf.d
/etc/httpd/conf.d/README
/etc/httpd/conf.d/proxy_ajp.conf
/etc/httpd/conf.d/welcome.conf
...
[download]

Is this what you were talking about, a runtime install...?

You'd have to download the source rpm (SRPM) to get the source they used. You can either do that, unpack it, and build mod_perl against it, or you can download the apache source, build apache and mod_perl, remove the apache rpm, and install your newly built apache in its place.