Since the script is old, you could try putting this code block at the end of your existing main code.

while( (scalar threads->list) > 0   ){
     print scalar threads->list,"\n"; 
     sleep 1;
     foreach my $thread (threads->list) {
        if( $thread->is_joinable ){ $thread->join;}
       }
  }
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You also should look at the old code, and see if die or exit is used anywhere. If any thread calls die or exit, it will bring down the whole program with your warning issued, which is harmless. However, you might want to put this in your code

use threads ( 'exit' => 'threads_only');
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