Just one thing with your program, as I recall with standard deviations you should have the average calculated over the whole data range, not just the elements that have come before.
Here is what I would do:

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;
[download]

Which produces records like

Smith:70:C
Jones:74:B
Rider:80:A
[download]