@beeqwop,
@Avrune—
Kyou Machiko (the mangaka) probably had something specific in mind, but there really isn't enough information here for the reader to know.
Sugar cane was and is grown on Okinawa, so presumably a healthy Okinawan girl would recognize it. But those two girls were in poor physical and psychological condition.
It may have been poisoned sugar cane, which is sometimes placed in traps for vermin. The girls got that piece from a corvid that may have got it from such a trap.