Something you should note with your hash:

Your @grades array has students with multiple grades. When you convert @grades to %grades, you are losing the first entry.

The following script

use Data::Dumper;

@grades = qw(Noel 25 
             Ben 76 
             Clementine 49 
             Norm 66 
             Chris 92 
             Doug 42 
             Carol 25 
             Ben 12 
             Clementine 0 
             Norm 66);

%grades = @grades;  

print Dumper \@grades;
print Dumper \%grades;
[download]

will output

$VAR1 = [
          'Noel',
          '25',
          'Ben',
          '76',
          'Clementine',
          '49',
          'Norm',
          '66',
          'Chris',
          '92',
          'Doug',
          '42',
          'Carol',
          '25',
          'Ben',
          '12',
          'Clementine',
          '0',
          'Norm',
          '66'
        ];
$VAR1 = {
          'Chris' => '92',
          'Carol' => '25',
          'Doug' => '42',
          'Norm' => '66',
          'Clementine' => '0',
          'Ben' => '12',
          'Noel' => '25'
        };
[download]

and you'll see that only the last entry for each student is stored in your hash (which is fine if that is what you want to do, but I thought I would point that out just in case you didn't realize that was happening).