The only thing I can see wrong, which may cause hanging, is the way you wait for the thread joins to occur.

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
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