This will output Total count : 5, because you only increment $total_count by one on each iteration. You probably want $total_count += $ret instead (or $total_count += $t->join, if you won't be using the return value for anything else, i.e.):

    $total_count += $_->join for threads->list;
    print "Total count : $total_count \n";
    __END__
    Total count : 15
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