use XML::Twig;
use IO::File;

my $twig = XML::Twig->new();
$twig->parsefile('20.xml'); # you can use parseurl if 20.xml really is remote.

my $localtxt = IO::File->new('20.txt', 'w') or die "Can't open 20.txt for writing: $!";

foreach ($twig->get_xpath('//permalink'))
{
  $localtxt->print($_->text(), "\n");
}
$localtxt->close();