You could use a hash and count within the loop:

my $previous_grade = '';
my %grade_count;
for my $item ( sort { $a->{'GRADE'} cmp $b->{'GRADE'} } @$data ) {
    my( $grade, $name ) = ( $item->{'GRADE'}, $item->{'NAME'} );
    print $grade eq $previous_grade ? ( ' ' x ( length( $grade )+1 ) )
+ : "$grade,";
    print "$name\n";
    $previous_grade = $grade;
    $grade_count{$grade}++;
}
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