I looked this up. I do not see future perfect/ Where did you get this information?
NT:4982 σῴζω (others, σῴζω (cf. WH. Introductory § 410; Meisterhans, p. 87));
future σώσω; 1 aorist ἔσωσα; perfect σέσωκα; passive, present σώζομαι;
imperfect ἐσωζομην
(from Thayer's Greek Lexicon, PC Study Bible formatted Electronic Database. Copyright © 2006 by Biblesoft, Inc. All rights reserved.)
Thayer doesn't show the future perfect; but I have a parsed interlinear (not previously consulted) which agrees with my parsing
σωθήσεται
V3 SFPI
You can plainly see that this is not a simple future.