use strict;
use XML::Rules;

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

$parser->filter(\*DATA);

__DATA__
<li>
  <p>
   Some <b>text</b>
  </p>
</li>
[download]

If you wanted to remove only <p> directly inside <li>, the code would look like this:

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>
[download]

The $_[2] is a reference to an array containing the names of the opened tags, so the condition just checks whether the enclosing tag of the currently processed <p> is <li>.