Not positive this is your issue, as it could be several things. In newer versions the installation has been made easier, but I found part of the configuration for apache to be a little off. If you look in your OTRS conf file (on CentOS it is /etc/httpd/conf.d/zzz_otrs.conf) on about line 14 you will see:
<IfModule mod_perl.c>
if your mod_perl is included as mod_perl.so this causes an issue, simply change it to
<IfModule mod_perl.so>