in reply to Msg.pm + UDP
This book was printed about 16 years ago and, as such, is now rather dated. Perl 5.4 was the current version around that time (see perlhist for specifics).
I had a quick scan through Chapter 13 (I assume that's what you're referring to) but neither g-pkg nor #New Server ... leapt out at me. Could you reference their page numbers and also give some indication of what you're having trouble understanding.
I did find the code for set_event_handler() (page 214). Most of the examples of its usage (e.g. connect(), disconnect(), send_later()) do not have a package name as the first argument. However, on page 205, you will find the call:
Msg->set_event_handler(\*STDIN, "read" => \&kbd_input);
I'm not sure if that helps. Again, some indication of what you're having trouble understanding would help in providing a better answer.