The only way you are going to have a division by zero error is if there are no elements in @grades for a given $student. Simple way to debug is to add a print after @grades = split... that goes something like

print "student: $student :: grades: " . $grades{$student} . "\n";
or
print "student: $student :: grades: " . join( ' ', @grades ) . "\n";
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and see if any of the data looks inappropriate.