foreach my $id (
    map { $_->[1] }
    sort { $a->[0] <=> $b->[0] }
    map { [ (split( "\t", $students{$_} ))[1], $_ ] } keys %students )
{
    printf "$id\t$students{$id}\n";
}

##</code><code>##

4873    Tom     2.9
236     Fred    3.2
1522    Susan   4.0

##</code><code>##

    # Create a list of student IDs by taking the sorted
    # list of anonymous arrays and returning the second
    # element of each.
    map { $_->[1] }

    # Sort the list of anonymous arrays (created below)
    # numerically, using the first element in the array
    # (the GPA).
    sort { $a->[0] <=> $b->[0] }

    # For each key in %students, create an anonymous array:
    # The first element is the GPA, obtained by taking
    # the second element of the list created by splitting
    # the value of $students{$_} on the tab, and the second
    # element is the student ID.
    map { [ (split( "\t", $students{$_} ))[1], $_ ] } keys %students )