Create your text layer.
Create a 100% black solid layer
Create a 100% white solid layer, make sure it is above the black layer.
Precomp the two solid layers (cmd/control + shift + c) and jump into the comp.
Add a 'venetian blinds' effect to the white layer and delete the keyframes that automatically accompany it.
Set the 'Direction' to -90deg, 'transition ...