against local programs (e.g. when setuid), although the impact is likely
small, as the program has to handle connection errors already-

Examples: C<PERL_ANYEVENT_PROTOCOLS=ipv4,ipv6> - prefer IPv4 over IPv6,
but support both and try to use both.  C<PERL_ANYEVENT_PROTOCOLS=ipv4>
- only support IPv4, never try to resolve or contact IPv6
addresses. C<PERL_ANYEVENT_PROTOCOLS=ipv6,ipv4> support either IPv4 or
IPv6, but prefer IPv6 over IPv4.

=item C<PERL_ANYEVENT_EDNS0>

Used by L<AnyEvent::DNS> to decide whether to use the EDNS0 extension
for DNS. This extension is generally useful to reduce DNS traffic, but
some (broken) firewalls drop such DNS packets, which is why it is off by
default.

Setting this variable to C<1> will cause L<AnyEvent::DNS> to announce
EDNS0 in its DNS requests.

=item C<PERL_ANYEVENT_MAX_FORKS>

The maximum number of child processes that C<AnyEvent::Util::fork_call>
will create in parallel.

=back

=head1 EXAMPLE PROGRAM

The following program uses an I/O watcher to read data from STDIN, a timer
to display a message once per second, and a condition variable to quit the
program when the user enters quit:

   use AnyEvent;

   my $cv = AnyEvent->condvar;

   my $io_watcher = AnyEvent->io (
      fh   => \*STDIN,
      poll => 'r',
      cb   => sub {
         warn "io event <$_[0]>\n";   # will always output <r>
         chomp (my $input = <STDIN>); # read a line
         warn "read: $input\n";       # output what has been read
         $cv->send if $input =~ /^q/i; # quit program if /^q/i
      },
   );

   my $time_watcher; # can only be used once

   sub new_timer {
      $timer = AnyEvent->timer (after => 1, cb => sub {
         warn "timeout\n"; # print 'timeout' about every second
         &new_timer; # and restart the time
      });
   }

   new_timer; # create first timer

   $cv->recv; # wait until user enters /^q/i

=head1 REAL-WORLD EXAMPLE

Consider the L<Net::FCP> module. It features (among others) the following
API calls, which are to freenet what HTTP GET requests are to http:

   my $data = $fcp->client_get ($url); # blocks

   my $transaction = $fcp->txn_client_get ($url); # does not block
   $transaction->cb ( sub { ... } ); # set optional result callback
   my $data = $transaction->result; # possibly blocks

The C<client_get> method works like C<LWP::Simple::get>: it requests the
given URL and waits till the data has arrived. It is defined to be:

   sub client_get { $_[0]->txn_client_get ($_[1])->result }

And in fact is automatically generated. This is the blocking API of
L<Net::FCP>, and it works as simple as in any other, similar, module.

More complicated is C<txn_client_get>: It only creates a transaction
(completion, result, ...) object and initiates the transaction.

   my $txn = bless { }, Net::FCP::Txn::;

It also creates a condition variable that is used to signal the completion
of the request:

   $txn->{finished} = AnyAvent->condvar;

It then creates a socket in non-blocking mode.

   socket $txn->{fh}, ...;
   fcntl $txn->{fh}, F_SETFL, O_NONBLOCK;
   connect $txn->{fh}, ...
      and !$!{EWOULDBLOCK}
      and !$!{EINPROGRESS}
      and Carp::croak "unable to connect: $!\n";

Then it creates a write-watcher which gets called whenever an error occurs
or the connection succeeds: