The <DATA> syntax in perl

Where can I find more about the following syntax in perl?

The connection between <DATA> and __DATA__ is unclear.

while (my $date_string = <DATA>) {
  chomp($date_string);
  next if not length $date_string;
  print "$date_string ist Unixtime ",
        $lang_date->str2time($date_string),
        " und ",
    $lang_date->time2str( '%d.%m.%Y %T (%Z)',$lang_date->str2time($date_string) ),
        "\n";
}

__DATA__
1.3.1999
1 Marz 1999
1. Marz 1999
1/3/1999