my %grades = (
  0 => "f", 40 => "e", 50 => "d",
  60 => "c", 80 => "b", 90 => "a",
  95 => "a+"
);

my $grade = 45;

my $letter;
for my $score_min (sort keys %grades)
{
  if ($grade >= $score_min)
  {
    $letter = $grades{$score_min};
  }
  else
  {
    last;
  }
}

print $letter;