Most biologists and informaticians are a little weak in the classical languages. Fortunately, google helps you catch up on any lessons you missed. The developers in Germany tend to say pair-SAY-oos, with the accent on the second of three syllables, which seems reasonably close to the Greek pronunciation given here and here. (This pronunciation is distinct from how Germans usually call the Greek hero, which is PAIR-soys.)

The English pronunciation uniformly puts the accent on the first syllable: PER-see-uhs, as in examples 1, 2, and 3. For the software there is no official pronunciation, so no one will tell you you are wrong, as long as you say it with confidence.