How about

use constant FB_SPACE => sub { ' ' };

$bytes = encode($charset, $chars, FB_SPACE);
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Update: Tested:

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

use constant FB_SPACE => sub { ' ' };

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

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