The corridor is a ring 1400 meters in diameter, at a reasonable pace you make one round every hour. Every twenty meters there's a narrow sconce in the wall, with an idol and a small votive light in front of it. The hero cannot see the whole corridor, but he can spot several lights in front of him in the darkness, and they describe two straight lines, converging in the middle.
He looks back, and he sees the same.
So he is sure the corridor is straight.
When he walks forward, the two large LCD screens (or magic constructs) that lurk just outside the torch's range move as well, displaying what an endless, straight corridor would look like were you to look at it from the hero's point of view.
This requires some Kinect-level magic to reliably tell where the hero's eyes are, but has the advantage of not needing any recognizable cues to thwart the perception, which could make the hero suspicious ("Why all this clutter? I almost can't tell whether the corridor is straight! Errr... hold that thought...").
A similar trick
using a grav engine and a corridor bending on the vertical plane
was pulled on a guy called Hulon in Theodore Sturgeon's What Dead Men Tell, and a wholly different principle was used on a larger scale in James P. Hogan's novel Endgame Enigma.
The corridor appears to be solid, and unmoving. It actually is neither. It is a racetrack-shaped running mill, three hundred meters long, built to tolerances small enough that near the hero it appears to be solid. One hundred and fifty meters behind the hero, the slices making up the corridor unlock, bend a full 180 degrees like baggege conveyor belts in airports, and are rolled back in the opposite direction. If the corridor is sufficiently soundproofed and the movement is smooth enough, the hero will notice nothing from the inside, and he'll be in the middle of a thousand yard corridor that is perfectly straight.
Magic (or technological) light bending
By supplying vertical laminar flows of air heated and cooled at different temperatures through grilles in the floor and ceiling - they can be deactivated by the hero's pressure on the floor, to provide a more comfortable environment while he walks - it is possible to bend the light so that the corridor appears to be straight, even with better lighting than a fire torch.
This is the same effect that makes the sky reflect on a road on a hot day.
Of course the corridor will appear to shimmer, but it's unlikely that the hero is conversant enough with physics (or vertical mirages) to cotton up to what's happening.