Hi,
I'm totally new at Linux, but I've received the task to install mod_perl on a Red Hat Enterprise Linux Server 5.3. Muy current Apache version is 2.2.3 and perl is 5.8.8, so I have to install mod_perl 2.0. I have tried to do it, but I get the following error on the "perl Markefile.PL" step of the process:

[root@xxx mod_perl-2.0.4]# perl Makefile.PL MP_APXS=/usr/local/psa/admin/bin/apxs
Reading Makefile.PL args from @ARGV
MP_APXS = /usr/local/psa/admin/bin/apxs
no conflicting prior mod_perl version found - good.
************* WARNING *************

Your Perl is configured to link against libgdbm,
but libgdbm.so was not found.
You could just symlink it to /usr/lib64/libgdbm.so.2.0.0


************* WARNING *************
[ error] Unable to open /usr/local/psa/admin/include/ap_release.h: No such file or directory
[ error] Unable to determine server version, aborting.
[ error] Invalid MP_APXS specified?

The "ap_release.h" file is missing, but the apache server is working just fine. I really don't know how to fix this, since I'd prefer not to mess too much with the current apache installation, since there are some important configurations already in place. Please help me...I would really appreciate it.

There should be a pkg available through yum or rpm to do that for you. Much easier than trying to do it by hand.
If you've paid for RHEL you can check the website via your rhn login.
If not, you could try Centos, although I wouldn't really recommend mixing them generally.