"were perished" is correct and usable in written English.
You can use it when talking about something that has been destroyed or lost entirely. For example, "Many of the city's oldest buildings were perished in the fire."
During this sad and moving weekend, when I will think about my dear ones, the innocents who were perished at the hands of the Nazis, I will be comforted by the wisdom of my children and my teachers.
A further survey in 1576 reported that, although the stonework was in good condition apart from one chimney and window, the timbers were perished, the lead roof was full of holes, one kitchen was on the verge of collapse, and another needed its floor replacing.
As if Nature herself were perishing".
Churchill was unmoved when millions were perishing in Bengal.
Most everybody desperately wanted to go home, and insects and other small life forms were perishing from boredom.
As most "pure play" e-commerce companies were perishing, the outsourcing model of Global Sports started gaining proponents.
He led expeditions to Egypt and the Sudan (1905 07) and copied inscriptions from monuments that had been previously inaccessible or were perishing.
His only reply to a telegram reporting how many Bengalis were perishing was to ask "why Gandhi hadn't died yet".
We sought to determine at what stage (embryonic or post-natal) the double mutants were perishing.
Some cells were perishing without succeeding in production of either proper erythrosomes or a lumen despite initiating those processes.
The cells were perishing in the process but at the same time, they were changing the microenvironment by leaving traces of their presence in the ECM as developmental clues for other cells.
