Due to the eastern monsoon, the season in the Perhentians (and all other east coast islands) is effectively limited to the period between the beginning of March or April until late October. Outside this period the seas can be very rough and choppy, skies overcast, and with currents that make swimming dangerous and most (though not all) accommodation options are closed. Do not believe the travel-agents when they claim the accommodations are open. Even if they will provide a room, restaurants and all shops (i.e. no place to get water, food, sunscreen etc.) are closed in off-season. If the restaurant is open, food choices will be very limited and over-priced.
When going during off-season be aware that there will be literally no one there but you and maybe a handful of locals.