It looks like ActiveState perl strikes again.

That strange, because I'm using ActiveState Perl also.

I guess it is blocking.

Sockets are blocking by default and nothing in the code you posted attempts to set them as non-blocking.

Before I start reading from the IO::Socket in the Client3 I can write to it. Once I start reading from it in Client3::get_message() what I write to the socket is never sent to the other side of the socket.

You cannot simultaneously read and write to a blocking socket. It's like using a walky-talky. There is no point in talking whilst the person on the other end is transmiting, because they will not hear you.

But I don't see the need for a non-blocking socket for implementing the SMTP protocol?

It is a command-response protocol. Your client3 code should work in the same way as the Client2 code. Wait for a command, and once you get one, make no more reads until it has finshed sending the complete response to that command.

I used a poor choice of words. Blocking isn't right. What I meant to say was that once I start reading from the socket (one line at a time) I can't write to it and have it received. I have switched back to the Client2 code. The offending portion is in get_message(). If I run with the following code:

# sub process {
sub get_message {
    my $self = shift;
    my($cmd, @args);
    
    my $sock = $self->{SOCK};
    $self->_reset0;
    
    print $sock "STUFF1\r\n";
    while(<$sock>) {
    print $sock "STUFF2\r\n";
    print "$$ command: $_";
... rest of routine
[download]

The output (from the perspective of the client is:

Net::SMTP>>> Net::SMTP(2.31)
Net::SMTP>>>   Net::Cmd(2.29)
Net::SMTP>>>     Exporter(5.60)
Net::SMTP>>>   IO::Socket::INET(1.31)
Net::SMTP>>>     IO::Socket(1.30)
Net::SMTP>>>       IO::Handle(1.27)
Net::SMTP=GLOB(0x20f758c)<<< 220 Debatable SMTP 0.2 Ready.
Net::SMTP=GLOB(0x20f758c)>>> EHLO localhost.localdomain
Net::SMTP=GLOB(0x20f758c)<<< STUFF1
[download]

In other words it only writes to the client the first time (it never sends STUFF2 to the client). Once the socket is read from writes aren't happening in other words. It is perplexing.