Perplex wouldn't affect the new defense value (remains 17).
Perplex' effect stop when the target takes damage.

From the PAC:
"PERPLEX Once during your turn (but not during another action), as a free action this character modifies by +1 or –1 any combat value (including range) of a target character until the beginning of your next turn (it can target itself). A character using this power must be within 10 squares of the target and have a clear line of fire to the target. This effect ends immediately if this character loses Perplex or is defeated, or when the target is damaged or healed."

EDIT:

Okay, this came up during one game.

You've got some Cuckoos surrounding Dr. Strange on his first click (18 d all around). They move, boosting their defense. Their new defense is an 18 or 20 (21 if surrounded by three other Cuckoos)?

Thanks Harpua and Drunky

Their defense is 18 if they are using Dr. Strange's defense, which is his unmodified defense.

Defend wouldn't be used until one of the Cuckoo is attacked. And at this time, if Defend is used, the replacement value coming from Strange would cover the modifier(s) coming from the Hive Mind special power.
Basically, you can't use both the replacement value from Dr Strange's Defend and the modifier from Hive Mind, it's one or the other.

EDIT: what Crow said