use IO::Handle;
use IO::Select;

use constant {
  BLOCKSIZE => 8192
};

my $c0 = ...; # Connection to client 0
my $c1 = ...; # Connection to client 1

# Make connections non-blocking
$c0->blocking(0);
$c1->blocking(0);

# Initialize receiving selector and buffer table
my $rcvsel = IO::Select->new($c0, $c1);
my %rcvbuf = ($c0 => '', $c1 => '');

# Setup target mapping
my %sndtgt = ($c0 => $c1, $c1 => $c0);

# Initialize sending selector and buffer table
my $sndsel = IO::Select->new();
my %sndbuf = ($c0 => $rcvbuf{$c1}, $c1 => $rcvbuf{$c0});

# Loop while data has to be received or sent
while ($rcvsel->count > 0 or $sndsel->count > 0) {
  my @ready = $rcvsel->can_read();
  for (@ready) {
    # If there is incoming data, determine receiving buffer and fill it
    my $buf = \$rcvbuf{$_};
    my $len = $_->sysread($$buf, BLOCKSIZE, length($$buf));

    if (defined $len) {
      if ($len > 0) {
        # If data was received, add the output target to the sending selector
        $sndsel->add($sndtgt{$_});
      }
      else {
        # At end of stream, remove the input from the receiving selector
        $rcvsel->remove($_);
      }
    }
    else {
      die "read error: $!";
    }
  }

  @ready = $sndsel->can_write();
  for (@ready) {
    # If the connection is ready for output, try to flush the buffer
    my $buf = \$sndbuf{$_};
    my $len = $_->syswrite($$buf);

    if (defined $len) {
      # If data was sent, remove it from the buffer
      substr($$buf, 0, $len) = '';

      if (length($$buf) <= 0) {
        # If the buffer is empty, remove the output target from the sending selector
        $sndsel->remove($_);
      }
    }
    else {
      die "write error: $!";
    }
  }
}

# All data has been transferred, so we close the streams
$c0->close();
$c1->close();