#!/usr/bin/perl

use strict;
use warnings;

use HTML::Parser ();
use HTML::Entities qw(encode_entities);

sub disable_tags_but {
    my ($text, $allowed_tags) = @_;

    my @text;
    my %allowed_tag = map { $_ => 1 } @{$allowed_tags || []};

    my $tag_h = sub {
        my ($tag, $text) = @_;
        $text = encode_entities($text, "<") unless $allowed_tag{$tag};
        push(@text, $text);
    };

    HTML::Parser->new(
        start_h   => [$tag_h, 'tagname, text'],
        end_h     => [$tag_h, 'tagname, text'],
        default_h => [\@text, '@{text}'],
    )->parse($text)->eof;

    return join("", @text);
}

#
# Test it
#
print disable_tags_but(<<EOT, [qw(a br)]) unless caller;

Test
<foo>  <a href="...">...</a>
</bar>

EOT