The word pereciera is the imperfect subjunctive form of perecer in the first person singular. There are other translations for this conjugation.
See the full perecer conjugation
Es de suponer que me asombré cuando conocí que no hacía falta una guerra mundial nuclear para que pereciera nuestra especie.
One can imagine my surprise when I learned that a nuclear world war was not needed for our species to disappear.
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