To expand what nikosv wrote: yes, I know this is stripped down example code - but in the child branch, you MUST exit

     foreach my $hostname (@host)
     {
         my $thr = fork();
         if($thr) {
             #parent
             push(@childsall, $thr);
             next;
         }
         else
         { 
            connecttohost($hostname);
            exit; # <---- here
         }
     }
      foreach my $child_pid (@childsall) {
         my $tmp = waitpid($child_pid, 0);
      }
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because otherwise each child resumes the loop at its current state, spawning child processes which spawn child processes which spawn child processes which spawn child processes which spawn...