The error message means you need to install Net::SMTP_auth. In addition:

my $smtp = Net::SMTP->new( ... );

... should probably be instead:

my $smtp = Net::SMTP_auth->new( ... );

... and you almost certainly don't need the single quotes in:

$smtp->auth('CRAM-MD5', '$ServerAccount', '$ServerPwd');

... if you want the values of the variables, and not the literal strings.