You can usually cut thin paneling with a utility Knife.
You can also use a an old fashioned hand saw and angle it into the corner; upward to the top,
downward to the bottom.
There aren't any powersaws that an average homowner has access to, that would do this cut, Maybe a careful run with a sawzall. A baseboard saw would do it, but even I don't have one of those!
I hope the paneling isn't 1/2" thick!
Welcome to the forum,