There's not much in the SIP protocol itself (I'm not talking the RTP streams here, just SIP) that can't be handled with adequate performance in higher level languages (whether interpreted or not). At the end of the day it's a bunch of XML over TCP sockets, a lot of your function/method calls are going to be into libc (networking), something like sax (xml), and likely database access (libpq/whatever), the coordination of these with perl is going to cost you some CPU time but likely not enough to justify the extra expensive (man hours) doing it in C or something similar.

The RTP streams, that you may want to handle in a lower level language since you're talking soft real time being a must.