I stand corrected:

use Parse::RecDescent;

my $parser;

$parser = Parse::RecDescent->new(q` test: { $return = 1 } `);
print($parser->test('') ? 'ok' : 'fail', "\n");  # ok

$parser = Parse::RecDescent->new(q` test: { $return = 0 } `);
print($parser->test('') ? 'ok' : 'fail', "\n");  # fail

$parser = Parse::RecDescent->new(q` test: { $return = '0' } `);
print($parser->test('') ? 'ok' : 'fail', "\n");  # fail

$parser = Parse::RecDescent->new(q` test: { $return = undef } `);
print($parser->test('') ? 'ok' : 'fail', "\n");  # fail

$parser = Parse::RecDescent->new(q` test: /\d+/ `);
print($parser->test('1') ? 'ok' : 'fail', "\n");  # ok
print($parser->test('0') ? 'ok' : 'fail', "\n");  # fail
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I never got into that situation, because I'd be returning
[ ident   => ident ]
or
[ literal => literal ]
or
[ '*'     => expr, expr ]
or
[ '/'     => expr, expr ]
or
...