You could use a neat little table

print $_,     $_ > 90 ? 'A'
            : $_ > 80 ? 'B'
            : $_ > 70 ? 'C'
            : $_ > 60 ? 'D'
            : 'F'
, "\n"
  for 89,90,91, 80, 70 , 50;
[download]

and if you wanted a hash, you could simply

my %grade_for = map { $_ => 
                      $_  > 90 ? 'A'
                    : $_  > 80 ? 'B'
                    : $_  > 70 ? 'C'
                    : $_  > 60 ? 'D'
                    : 'F'
                    } for 0..100;
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... later

  $student_name = 'f00li5h'; $student_score = 61;
  print "$student_name scored $grade_for{$student_score}!"
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here I've used map to create a lookup. My %grade_for is the same as your %scores, although perhaps an array would have worked just as well here (due to the non-sparse numeric keys)

f00li5h nudges the grammar