Given this

package Msg;
sub connect {
  my ($pkg, $to_host, $to_port, $rcvd_notification_proc) = @_;
}

# ...

Msg->connect($host, $port, \&rcvd_msg_from_server);
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The connect method is not being called with three arguments, it's being called with four arguments. The first argument is the string "Msg" - that is, the part before the ->. This is how method calls work in Perl (we borrowed it from Python); the thing before the arrow gets passed in as the first argument.

"From the comment, an object was created. The reference is the anonymous hash with 2 members, and the class name is what? $pkg appears to be an undefined scalar?"

$pkg is "Msg".

"From the prior 'bless', a $conn has no methods"

Why do you think $conn has no methods? Something to do with the hash only having a couple of uninteresting key-value pairs? An object's methods have nothing to do with the blessed reference itself, but are taken from the package it is blessed into.

So because $conn is blessed into the Msg package, all the subs defined within Msg are available as methods for $conn.

"Why do you think $conn has no methods? "
Because of my ignorance of the nuances of Perl classes and objects. All of the texts I looked at started with a package and a sub called 'new' to create the object. I had been looking at Msg.pm as a way to carve out and localize a set of functions and have a cleaner organization. That is the only usage I knew about for packages.

This exploration has been enlightening, and gives me a new light to look at Perl packages. The simple explanation of:

The first argument is the string "Msg" - that is, the part before the ->. This is how method calls work in Perl (we borrowed it from Python); the thing before the arrow gets passed in as the first argument.

Is the key that I never found in my frantic scanning of my library; I didn't find an explanation of the mechanics, just the conceptualization of OO.

Thanks a bunch! odd, ending my text with an exclamation point messes up formatting.

!-- Node text goes above. Div tags should contain sig only -->

It is always better to have seen your target for yourself, rather than depend upon someone else's description.

"All of the texts I looked at started with a package and a sub called 'new' to create the object."

It is convention to name constructors new but there's no requirement to do so. You could just as well call it old or frangipane and it would make not one whit of difference to the Perl interpreter. But the reason that you should call a constructor new (and not old or frangipane) is for humans reading your code. When they see new they'll know precisely what that method call is doing!

That said, it's quite common for a class to define additional constructors such as new_from_json or new_from_yaml in addition to their standard new constructor. These are often wrappers around new that do additional pre-processing or post-processing.

package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

Whenever you see a construct that looks like Module->subroutine you should think "Class Method". The subroutine will be called with the Class name as the 1st parameter, and as expected it's used in a bless command.

Now that $conn is an object, all subroutines in it's class' (Msg) namespace and in the namespaces listed in Msg::ISA, can now be used as methods. Any such method will receive the blessed reference $conn implicitly as it's 1st parameter.

I went back and looked at all the subroutines.
2 have $pkg as arg0 (event_loop() & new_server())
7 have $conn as arg0
3 don't take args
1 starts with the line:

  shift unless ref($_[0]); # shift if first arg is package name
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So maybe the entire package is actually a class.

With 1 dodgy routine:

sub _new_client {
    my $sock = $main_socket->accept();
    my $conn = bless {
        'sock' =>  $sock,
        'state' => 'connected'
    }, $g_pkg;             #< different CLASSNAME
    my $rcvd_notification_proc =
        &$g_login_proc ($conn, $sock->peerhost(), $sock->peerport());
    if ($rcvd_notification_proc) {
        $conn->{rcvd_notification_proc} = $rcvd_notification_proc;
        my $callback = sub {_rcv($conn,0)};
        set_event_handler ($sock, "read" => $callback);
    } else {  # Login failed
        $conn->disconnect();
    }
}
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It is always better to have seen your target for yourself, rather than depend upon someone else's description.

Common Perl convention is that sub/method names beginning with an underscore are to be treated as private. The reason that sub _new_client is structured differently than the other subs in the package is because it's meant for internal use only and is not a part of the public interface.

Let me try to toss-in an explanation of the big picture here:

(1)   In Perl, an “object” is (usually...) a hashref that has been passed to the built-in function, bless, which takes as its (usually...) two parameters the hashref to be blessed and a package name.   Internally, Perl now flips a switch that says that it is “blessed,” and associates the specified package-name with it.   (Yes, it can be, and occasionally is, re-blessed, which changes the associated package-name.)

Any piece of code which serves this function, no matter where it is located nor what it is named, serves this purpose.   Specifically, there is nothing “magical” about a function named new.

(2)   When something has been blessed, it becomes possible to use the $obj->methodname(args...) syntax.   This simply translates to a subroutine call, to a sub (maybe...) found in the previously-associated package, but passing $obj as an additional, first, parameter.

You will therefore find that these subroutines begin with a statement such as my $self = shift;, which grabs that first-parameter that has been added.   Once again, the name $self is simply a convention.

HTH...   If you are used to looking at very formal, computer-science-y language implementations, you might be a bit startled by Perl.

Use the POD, Wiggins!

If you're on a Mac or Linux/Unix system, type into a terminal window:

perldoc perlobj
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to read about using objects in Perl. If you can't use a terminal window, or you're on Windows, go to http://perldoc.perl.org/perlobj.html

There are tons (ie several dozen) of tutorial/reference files installed on your system; perldoc perl contains the entire list. perlootut is a great OO guide for beginners.