This reply is to address the change if the modification takes place in MIME::Lite vs. Net::SMTP as my previous post suggests. I think the change in behavior should occur at the send method rather then the send_by_smtp. This would allow future modifications of send methods to accounted for in the same manner. So here is my proposed revised sub send:

# add new global that the top
my $SenderOpts = '';


sub send {
    my $self = shift;

    if (ref($self)) {              ### instance method:
        my ($method, @args);
        if (@_) {                            ### args; use them just t
+his once
            $method = 'send_by_' . shift;
            @args   = @_;
        }
        else {                               ### no args; use defaults
            $method = "send_by_$Sender";
            @args   = @{$SenderArgs{$Sender} || []};
        }
        $self->verify_data if $AUTO_VERIFY;  ### prevents missing part
+s!
        return $self->$method(@args);
    }
    else {                         ### class method:
        if (@_) {

            my @old = ($Sender, @{$SenderArgs{$Sender}});
            $Sender = shift;

            if ( ref($_[0]) eq 'HASH') {
                $SenderOpts = shift;
            }
            $SenderArgs{$Sender} = [@_];    ### remaining args
            return @old;
        }
        else {
          Carp::croak "class method send must have HOW... arguments\n"
+;
        }
    }
}
[download]

Then below in our send_by_smtp method we add:

    if ($SenderOpts) {
         $smtp->auth( $SenderOpts->{auth_username}, $SenderOpts->{auth
+_password} );
    }
[download]

directly after our $smtp object is created.