Off of the top of my head; untested:

use HTML::Entities;
use HTML::TokeParser::Simple;

my $p = HTML::TokeParser::Simple->new('somefile');

while ( my $token = $p->get_token ) {
    print $token->as_is;
    next unless $token->is_start_tag( 'textarea' );
    while ($token) {
        $token = $p->get_token;
        last if $token->is_end_tag( 'textarea' );
        print HTML::Entities::_entities( $token );
    }
    print $token->as_is;
}
[download]