From pero +‎ -aja.

perája f (Cyrillic spelling пера́ја)

1. fin, flipper (appendage of a fish)
2. swim fin, flipper (finlike swimming accessories worn on the feet)

• perajar

• Skok, Petar (1972), “pèro”, in Etimologijski rječnik hrvatskoga ili srpskoga jezika [Etymological Dictionary of the Croatian or Serbian Language] (in Serbo-Croatian), volumes 2 (K – poni¹), Zagreb: JAZU, page 641
• Vukušić, Stjepan; Zoričić, Ivan; Graselli-Vukušić, Marija (2007), Naglasak u hrvatskome književnom jeziku (Velika hrvatska gramatika; 4), Zagreb: Globus, →ISBN, §259, page 75