I believe perpetualis and the rest of the Defiance weapons are still dropping from the Avalon exotic mission. Go to Avalon in the Pinnacle playlist and select legacy drops option in the customize tab. Fair warning, Avalon is one of the harder missions so I recommend not worrying about the other drops and just slapping on a bunch of positive modifiers

As neonsox42 says you can get it from the Avalon mission. You can farm that mission once per week for 2 guaranteed red borders on the first run, but that can be any weapon in its loot pool so it will take you awhile before you get 5 red borders for the crafting pattern. If you're running it solo, select the lowest difficulty and make sure you add "classic collector" in the boons modifiers so it gives you the red borders, it's worth adding all the other boons also such as brawler, grenadier and full throttle as the difficulty doesn't matter for the red borders.