-they perished
The word pereciesen is the imperfect subjunctive form of perecer in the third person plural.
See the full perecer conjugation


Tú me dijiste que estuviese pendiente de cualquier actividad en las calles que pereciesen extrañas, ¿cierto? Es correcto.
You told me to be on the lookout for any activity on the street that seemed strange, right? That's right.
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