After looking through the code, and then astutely reading the README, I saw the following:

    If you are installing the server, you need to put in in inetd.  A 
+line like this one should cut the mustard:

nrpep  stream  tcp     nowait  nobody    /usr/sbin/tcpd /usr/sbin/nrpe
+p -c /usr/local/nagios/etc/nrpep.cfg

    (All one line)

    The "nrpep" part is an entry in /etc/services for the port that nr
+pep needs to run on.  The "default" is 8086.  I pulled it out of my a
+ss.  If it conflicts with anything, let me know.  Otherwise, that's t
+he default.
An example:

nrpep        8086/tcp
[download]

So, although I haven't done any socket programming using inetd, I'm going to make a big assumption that inetd is in fact handling the listening server socket and passing the info to what appears to be <STDIN> and <STDOUT> for the nrprep server.

In looking at the check_nrprep code, it does a use Socket; right near the start. The client opens a socket connection and goes out to the server that is already listening via inetd.

Hope that explains it (also hope I was right!)