See Simple threaded chat server. Threads can share socket filehandles thru the fileno. Look at the multi-echo chat example. You can share the clients in a shared array.

So all you would need to do is create another thread for your keepalive signal, and put in it

 foreach my $fn (@clients) { 
      open my $fh, ">&=$fn" or warn $! and die;
       print $fh  "keepalive\n"  
  }
[download]