use strict;
use XML::Rules;

my $parser = XML::Rules->new(style => 'filter', rules => {
	_default => 'raw',
	p => sub {
		if ($_[2][-1] eq 'li') {
			return $_[1]->{_content};
		} else {
			return $_[0] => $_[1];
		}
	},
});

$parser->filter(\*DATA);

__DATA__
<root>
<li>
  <p>
   Some <b>text</b>
  </p>
</li>
  <p>
   Other <b>text</b>
  </p>
</root>