Okay! I’m against Sharingan Theory.
But we can see Zoro has a scar.
His eyelid is closed. Think!
Eyelid is naturally closed when someone try to strike this people’s eye.
We don’t see any scar else than the vertical scar.
Here’s I think what happen.
Mihawk train Zoro to get his own style and that is Tornado moves (sorry…forgot the name).
Because his tornado become uncontrollable then Zoro is training to close his eye to improve his accuracy of tornado moves.
Zoro says that he already know to read location of non living things (pretty sounds like CoO huh? ;p).
Then Mihawk train him to close one eye by fighting him. We all know that Mihawk is able to do a vertical attack from far away, and the attack is like a vertical sharp wind (see his first met with Zoro).
At first Mihawk train him with his GIANT sword (too big to be a sword huh?;p).
But of course Zoro knows to dodge it with his ability to hear non living things (see his fight with Mr. 1).
Then Mihawk train him with his SMALL sword (too big to be called as a sword huh? ;p).
At first Mihawk attack Zoro by doing simple attack.
Then he shows Zoro by attack things with vertical wind attack, he’s saying that he’ll attack like that.
Zoro was too afraid and keep dodging it.
But then Zoro understand to close his eyelid (advancing from Alabasta fight with Mr. 1).
And he attack Mihawk, Mihawk doing simple attack. But then he make his vertical attack.
DON! Zoro stop Mihawk with his sword.
DON! Vertical blood is in Zoro’s left eye.
It’s happen because the wind effect.
^^v how about this?