use constant FB_SPACE => sub { ' ' };

$bytes = encode($charset, $chars, FB_SPACE);

##</code><code>##

use Encode         qw( encode FB_DEFAULT );
use HTML::Entities qw( decode_entities );

use constant FB_SPACE => sub { ' ' };

my $chars = decode_entities('abc&hearts;def');
my $charset = 'US-ASCII';

for my $check (FB_DEFAULT, FB_SPACE) {
   print encode($charset, $chars, $check), "\n";
}

##</code><code>##

abc?def
abc def