Are you sure the SMTP server name didn't change at the same time? I get the error message "Can't call method "auth" on an undefined value at noname.pl line 6." when I run the following code:

use warnings;
use strict;
use Net::SMTP;

my $smtp = Net::SMTP -> new('wibble');
$smtp -> auth('URI', 'PSWD');
$smtp -> quit();
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which strongly implies to me that the new failed and $smtp is undef.

The use strict; ... remark is like a standard disclaimer around here :)