package Net::SMTP::Server::Client2::MySubClass;
use strict;
use vars qw/@ISA/;
use Net::SMTP::Server::Client2;
@isa = qw/Net::SMTP::Server::Client2/;

my %private_commands = ( HELO => \&my_hello,
                         [...]
                       );

sub new {
   my ($class, $sock) = @_;
   my $self = $class->SUPER::new($sock);
   bless $self, __PACKAGE__;
}

[...]

sub get_message {
   my ($self, $cmd, @args) = @_;
   return $self->SUPER::get_message($cmd, @args) unless exists $private_commands{$cmd};
   # from here on mimic the super class's method, but with 
   # calls to your private, "overridden" methods.

}

sub my_hello {
   # perform some individual stuff.
}