I would very much say Perl and IS/IT tie very well together. I'm a Security Analyst for a small firm and code
most of the inhouse IDS utillities. From Port Scanner to Network Analyzer and several common network tools like DNS Enumeration and Whois etc. Perl being 100% cross platform ? Lets us use my utillities on every and any machine on our open architecture LAN.
There is no reason why you cannot and should not continue to develope in perl and do IS as well. In fact it would be wise to pursue IDS development now while you're gaining IS/IT hands on experience.
x1b