There does seem to be a problem in multiple keys sort:

    $last[$a] cmp $last[$b]
    or
    $first[$a] cmp $last[$b]
[download]

should be:

my @index = sort {
    $last[$a] cmp $last[$b]
    or
    $first[$a] cmp $first[$b]
} 0 .. $#first;

     sort on multiple keys
Adler, Rich
Jackson, Jon
Jones, Fred
Jones, John
Smith, Joe
Smith, John

      But, testing, single sort (cmp):
     Fred
     Joe
     John
     John
     Jon
     Rich
[download]