use warnings;
use strict;
use XML::Twig;

my $str = <<EOF;
<li>
  <p>
   Some <b>text</b>
  </p>
</li>
EOF

my $t = XML::Twig->new(
    twig_handlers => {li => \&li},
    pretty_print  => 'indented',
);
$t->parse($str);
$t->print();

sub li {
    my ($t, $elt) = @_;
    for my $p ($elt->children('p')) {
        for my $c ($p->cut_children()) {
            $c->paste($elt);
        }
        $p->delete();
    }
}

__END__

<li><b>text</b>
   Some </li>