..Ive tried explicitly setting STDOUT to utf-8 via binmode..
Actually, your STDOUT should have been set to use ":encoding(UTF-8)" instead of "utf-8". Why?

":utf8" just marks the data as UTF-8 without further checking, while ":encoding(UTF-8)" checks the data for actually being valid UTF-8

this works for me though:

use warnings;
use strict;
use utf8;
use LWP::UserAgent;
use HTML::TreeBuilder;

my $url =
'http://buyingguide.winemag.com/catalog/peju-1998-reserve-cabernet-sau
+vignon-napa-rutherford';

my $browser = LWP::UserAgent->new;

my $re = $browser->get($url);

if ( $re->is_success ) {
    my $tree = HTML::TreeBuilder->new;
    $tree->parse( $re->decoded_content );
    $tree->eof();
    binmode STDOUT, ":encoding(UTF-8)";
    my $review_et = $tree->look_down( 'itemprop', 'reviewBody' );
    my $str = $review_et->as_text;

    #print $str,$/;  # this as works
    print $review_et->as_HTML;
    $tree->delete;
}
else {
    die $re->status_line();
}
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