Perfect IVs aren't necessary to achieve a 50 win streak in the Battle Subway, but it helps given that (I believe) the Battle Subway uses hex flawless Pokémon. I have achieved a 76 win streak in Super Doubles before with a team that had no perfect IVs whatsoever (lost at battle 77, because freeze hax stopped my Jellicent from setting up Trick Room), so I can assure you it's not a requirement to achieve a 50 win streak.
okay thank you. glad of that, cause i'm not that skilled at battling/breeding yet. (barely won the PWT in my Black 2 game)