This isn't very elegant (and maybe not too efficient), but it will store and extract the data in a way that is usable. I don't do html work, so you are on your own for figuring that part out :)
Note: it assumes that in milestones.xls, all the taskID's are stored in a single cell.

use Spreadsheet::ParseExcel::Simple;

my (%milestones, %taskID);
my $xls = Spreadsheet::ParseExcel::Simple->read('milestones.xls');
foreach my $sheet ($xls->sheets) {
    while ($sheet->has_data) {  
        my @data = $sheet->next_row;
        next if $data[0] =~ m/ID/;
        next if $data[0] eq '';
        my @tasks = split(",", $data[1]);
        foreach my $task (@tasks) {
             $milestones{$data[0]} = [ @tasks ];
        }
    }
}

my $xls = Spreadsheet::ParseExcel::Simple->read('TaskLists.xls');
foreach my $sheet ($xls->sheets) {
    while ($sheet->has_data) {  
        my @data = $sheet->next_row;
        next if $data[0] =~ m/ID/;
        next if $data[0] eq '';
        my @tasks = @data[2..3];
        $taskID{$data[0]} = [ @tasks ];
    }
}

foreach my $milestone (keys %milestones) {
    my @tasks = @{ $milestones{$milestone} };
    foreach my $task (@tasks) {
        print "milestone = $milestone, id = $task, start = @{ $taskID{
+$task} }[0], stop = @{ $taskID{$task} }[1]\n";  
    }
}

exit;
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