Hey funny !
I take part in this survey...
(Perl was of course among my choices... ;-)
(With nessus, snort, tripwire, crack and some other less known)

Now to answer your question :

I use Perl to make my private security tools
(private only beccause they're not coded well enough for a public release ;-)

Existing Perl modules allow you to focus on what you're trying to do and not waste time reinventing the wheel.
(Net::RawIP allowed me to code a stealth portscanner quite easily...)

The regex are a big plus for data manipulation/decoding too
(one of my very first Perl script was a filter taking a tcpdump output and giving me POP3 passwords...)

And last but not the least, nothing can match Perl to 'glue' easily several existing programs,
that's what I tend to do when playing with security/administration.

"Only Bad Coders Badly Code In Perl" (OBC2IP)