Like zaxo points out, buffer overflows in Perl are very unlikely, since Perl doesn't force array/hash limits. If you want to add 10^100 elements to an array, just make sure you have enough memory. There are a number of resources which can help you on secure programming (in general). Check perlsec for starters. Some bits on Perl & CGI security are here, here and here. It helps to turn on warnings (discussed in perlvar, perlrun and perldiag) and taint checking (check here for more on it).

Greetz
Beatnik
... Quidquid perl dictum sit, altum viditur.