foreach(@array){ 
   push @threads, threads->create(\&execute_ls, $_); 
} 

# when you loop thru @threads to join, join will wait for each thread
to finish and becoming joinable
   foreach (@threads) { 
       $_->join(); 
}
# better would be
  foreach (@threads) { 
      if ( ( $_ -> is_joinable) {  $_->join(); } 
}

alternatively you can detach your threads, instead of 
joining them, and write your return values to shared variables