Any service open to the Internet at large must take security into account. Large, complex software tends to expose subtle vulnerabilities that attackers can exploit to gain unauthorized access to the server host. Third-party modules or libraries can also contain similarly exploitable bugs. Perl scripts aren't immune either: incorrect untainting and sanitizing of user input can lead to disaster when this input is fed to the open( ) or system( ) functions.
Also, if the same mod_perl server is shared by more than one user, you may need to protect users of the server from each other (see Appendix C).