How would the seasons be affected if the Earth's axis were not
inclined 23 1/2 degrees from the plane of the ecliptic, but were
instead perpendicular to that orbital plane?

There would be no seasons.  The equatorial area could be come warmer
from constant maximum "summer" exposure to the sun.  Norhtern and
southern temporate areas would probably become cooler from lack of
summer warming.  It could have an effect on major ocean currents, but
I can't venture to suggest what these would be.