use strict;
use warnings;
use HTML::TokeParser;

my @tags = @ARGV;
@tags || die "Give a list of tags to retain.\n";
my %keep = map { lc($_) => 1, lc("/$_") => 1 } @tags;

my $p = HTML::TokeParser->new(\*DATA);

while ( my $t = $p->get_token )
{
    if ( $t->[0] =~ /S|E/ and $keep{$t->[1]} )
    {
        print $t->[-1];
    }
    elsif ( $t->[0] eq 'T' )
    {
        print $t->[1];
    }
}

__DATA__
<div>
    <h1>Bang!<sup>1</sup></h1>
    <p>Did <i>italic</i> and <a href="/uri">link with <b>bold</b>
       inside it</a>.</p>
 <a href="/top-level">naked link</a>
    <p><i>The</i> <b>content</b> of the body <sup>element</sup> is
       displayed in your <span>browser</span>.</p>
</div>