$students{0354} = "Fred\t3.2";
$students{4873} = "Tom\t2.9";
$students{1522} = "Susan\t4.0";

##</code><code>##

foreach $key(sort GPA_sort %students)
{
     #etc
}

##</code><code>##


sub GPA_sort
{
     $value_a = $students->{$a};
     $value_b = $students->{$b};
     ($name_a, $gpa_a) = split(/\t/, $value_a);
     ($name_b, $gpa_b) = split(/\t/, $value_b);

     $gpa_a <=> $gpa_b
}