I've recently started playing the turn-based strategy game Into the Breach. I have read online that the game is considered to have 'perfect information'. What is meant by 'perfect information' in this context?
A game with 'perfect information' refers to the fact that each player has all the available information through the entire duration of the game. A classic example of such a game is chess. Into the Breach is very similar to chess, in that regard, because it continuously shows the location and state of the all the units for both the player and the opponent.

However, it is debatable whether Into the Breach qualifies as a game with perfect information, because some information is still hidden from the player. An example of this is enemy units that spawn in later turns of the game. It is not clear to the player when or where these enemies will appear.
Thats an interesting point, but I don't think I fully understand. 

We know where the enemy unit will spawn one turn before they spawn in, why is this a violation of perfect information? I'll know where they spawn after all.