If god's creation was perfect, how did humanity fall? Why was there another being tempting them to disobey?

Surely a perfect creation wouldn't attempt to persuade humans to disobey god.
Surely a perfect creation wouldn't, in the face for such persuasion, decide to disobey god.

I think the easy answer is "Free will! Free will!" but a perfect creation acting under its own free will would understand the correct course and take it.
Only a flawed creation could do otherwise.

So...was god's creation perfect? If so, how?