See Re: Ask for STDIN but don't pause for it and the last example. This is untested in your code, but something like this will work. Just add \*STDIN to your Select object and test for it.

# Add listen sockets to select
my $sockets = new IO::Select();
$sockets->add(\*STDIN );
$sockets->add($stream1_lsn);
$sockets->add($client1_lsn);
$sockets->add($client2_lsn);

.....

#Go into infinite loop, handling connections - how do I end this?
while(my @ready = $sockets->can_read) { #got data
  
foreach my $socket (@ready) { # find which socket sent data
     
    if ($socket == \*STDIN) {  
            #read stdin and detect a q;
            exit;  }
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