Apart from the fact that $smtp is scoped out of existence and Noauth being tested for defined looks like a tautology in this snippet, I would be inclined to factorise out the "unless" in such a situation where most of the lines of code in the two branches are identical, and use a tertiary operator instead :

my $smtp = MIME::Lite->send('smtp',
               $host,
               defined( $something_else )
                ? AuthUser => $user,
                  AuthPass => $pass
                : Noauth => 1,
               Debug    => 1 );
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