Ok, this guy really helped me out and here's what he wrote: 
my @data;
my $avg;

while (<IN>) {
  chomp;
  my $tmp = [split /:/];
  $avg += $tmp->[1];
  push @data, $tmp;
}

$avg /= @data; 

my $std = 0;
for (@data) { $std += abs($_->[1] - $avg) }
$std /= @data;

open(OUT, ">pa5c.dat") || die ("Can't open file $!\n");
for (@data) {
  my $grade;
  my $dev = ($_->[1] - $avg) / $std;
  if ($dev >= 1) { $grade = 'A' }
  elsif ($dev >= (1/3)) { $grade = 'B' }
  elsif ($dev >= (-1/3)) { $grade = 'C' }
  elsif ($dev > -1) { $grade = 'D' }
  else { $grade = 'E' }
  print OUT join(':', $_->[0], $_->[1], $grade) . "\n";
}
close OUT;

Which produces records like 
Smith:70:C
Jones:74:B
Rider:80:A

[download]
But now when I run the program, it says: Illegal division by zero at $avg /= @data; Does anyone know whats happening here?

In reply to A little problem by yabba

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