If you're talking about vuln scanner, I'd suggest :
nessus (my favourite one), or even whisker (written in Perl)...

If you're talking about detecting intrusion, a search with IDS as keyword will bring you tons of results :
Portsentry, Snort, Hostsentry, NFR, Tripwire, Toby...

Anyway a list of excellent security tools is available here (thanks to fyodor).

"Only Bad Coders Code Badly In Perl" (OBC2BIP)