With the attacking king in front of its pawn on the seventh rank and the defending rook on the long side, checks from a distance never end: wherever the king goes, it either loses the pawn or walks back into the checks.
White's king is on d7 beside the e6-pawn and would like to push it to e7 and e8. Black's king on g7 is cut off from the pawn but is on the short side, and Black's rook on a1 is on the long side with plenty of files between it and the white king. That is the perfect set-up for a perpetual.