Just to confirm (since the docs only mention this working with new and the parent wasn't confident in his answer), here's a working example of $::RD_AUTOACTION being set when Precompile is called:

use Data::Dumper      qw( Dumper );
use Parse::RecDescent ();

{
   local $::RD_AUTOACTION = q { [@item] };

   Parse::RecDescent->Precompile(
       'parse: /a/ /b/ /c/',
       'Grammar'
   )
      or die("Bad grammar\n");
}

{
   # Normally, this would be in another file.
   require Grammar;
   my $parser = Grammar->new();
   print(Dumper($parser->parse('a b c')));
}
[download]

outputs

$VAR1 = [
          'parse',
          'a',
          'b',
          'c'
        ];
[download]

If you comment out autoaction, the output is

$VAR1 = 'c';
[download]