I'm making some progress. I was able to get the pop_server plugin to start responding to client commands after making the following change. (I also switched from port 8110 used above to 1110 so I could test with Google's Gmail pop3 client.)

Line 442 in pop_server.pm changed from:
my @cmd = ($CFG->{location}->{path}->{perl}, "$CFG->{location}->{path}->{batch}/tools/gpopd.pl --inetd-service --nodaemon");
to
my @cmd = ($CFG->{location}->{path}->{perl}, "$CFG->{location}->{path}->{batch}/tools/gpopd.pl --port=1110 --service start");


It's better but still not working properly though. I added some debug logging code at a few different spots in pop_server.pm. It seems like the problem is with the
print '.', $EOL;
statements. These appear many times in the code and indicate that a multiline response is complete. For some reason clients like Outlook Express and Thunderbird are hanging/experiencing timeouts on multiline output commands. Outlook Express is having a problem with RETR and Thunderbird is having trouble with LIST. That print statement appears very basic. I don't know why it would be the culprit.