#!/usr/bin/perl -w
# vim: filetype=perl
use strict;
use HTML::Parser;
use HTML::Entities;

my @tags = map {('(?:\A' . $_ . '\z)')} qw(br p font h[1-6] a);
my $tag_RE;
{
    local $" = '|';
    $tag_RE = qr/@tags/;
}
my $unsafe = '^\w\s'
my $p = HTML::Parser::->new(api_version => 3);
$p->handler(start => \&tag_filter, "tagname, text");
$p->handler(end   => \&tag_filter, "tagname, text");
$p->handler(default => sub {print encode_entities(shift,$unsafe)},
                       "text");

sub tag_filter {
    print $_[1] if ($_[0] =~ $tag_RE);
}

local $/;
$p->parse(<>);
[download]

Update: I realized that since Ovid seems to want pretty much any special character escaped, it made much more sense to use the negated character class in $unsafe than to have the line noise of all of those special characters and be worried about missing one. It also avoids the typo of having an unescaped `-' in the original that causes all capital letters to be escaped.

-dlc