Notice the laying of the rests at the beginning of the line: if written one above the other, they would have meant a perfect long.
After the first perfection, we notice a sequence made of 4 2 rythms (in minim units), arising from imperfection of each breve by the semibreve on its right side. Such a sequence lasting for a while is, in itself, an indication of perfect time (remark taken from Apel).
This was indeed a tiny example, just for fun...
