use Fcntl qw/F_GETFL, F_SETFL, O_NONBLOCK/;

   $flags = fcntl(SOCK_HANDLE, F_GETFL, 0) 
       or die "Can't get flags for socket: $!";
   $flags = fcntl(SOCK_HANDLE, F_SETFL, $flags | O_NONBLOCK)
       or die "Can't set flags for socket: $!";