Perl's -T switch enables taint mode. In taint mode, Perl performs some checks on how your program is using the data passed to it. For example, taint checks prevent your program from passing some external data to a system call without this data being explicitly checked for nastiness, thus avoiding a fairly large number of common security holes. If you don't force all your scripts and handlers to run under taint mode, it's more likely that you'll leave some holes to be exploited by malicious users. (See Chapter 23 and the perlsec manpage for more information. Also read the re pragma's manpage.)
Since the -Tswitch can't be turned on from within Perl (this is because when Perl is running, it's already too late to mark all external data as tainted), mod_perl provides the PerlTaintCheck directive to turn on taint checks globally. Enable this mode with:
anywhere in httpd.conf (though it's better to place it as early as possible for clarity).
For more information on taint checks and how to untaint data, refer to the perlsec manpage.