->can_read(0) will only return handles that have been set into the IO::Select object. Only set those you are interested in.

BTW, from the fragment you gave, $sock is your listen socket and is only used for accepts. $client_sock is your read/write socket.

typical code fragment:

for my $handle ( $sel->can_read(0) )
  {
  if( $handle == $sock )
    {
    # do accepts here...
    }
  elsif( $handle == $client_sock )
    {
    # do sysreads here...
    }
  }
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