use warnings;
use strict;

print html_filter(<<'END_HTML', qw/pre strong i/), "\n";
aaa<pre>bbb</pre>ccc<www><i>ddd</i><strong>eee</strong>fff</www>ggg
END_HTML

use Mojo::DOM;
sub html_filter {
	my $html = shift;
	my %allowed = map {$_=>1} @_;
	my $walk; $walk = sub {
		my ($in, $out) = @_;
		for my $n ( @{ $in->child_nodes } ) {
			if ( $n->type eq 'cdata' || $n->type eq 'text' )
				{ $out->append_content($n->content) }
			elsif ( $n->type eq 'tag' ) {
				if ($allowed{$n->tag}) {
					my $t = $out->new_tag( $n->tag, %{$n->attr} )
						->child_nodes->first;
					$walk->($n, $t);
					$out->append_content($t);
				} else { $walk->($n, $out) }
			} # ignore other node types for now
		}
		return $out;
	};
	return $walk->(Mojo::DOM->new($html), Mojo::DOM->new)->to_string;
}

__END__

aaa<pre>bbb</pre>ccc<i>ddd</i><strong>eee</strong>fffggg