After digging into the Parse::RecDescent docs, I found out that the mechanism to alter Parse::RecDescent's preference for the first match is the <score: ...> directive. The code below illustrate how it solves your case.

use strict;
use warnings;
use Parse::RecDescent;

my $grammar = q/
  startrule: choice  "a"
  choice:   "a" "a" <score: -2>
          | "a" <score: -1>
    /;
my $parser  = Parse::RecDescent->new($grammar);

while (<DATA>) {
    chomp($_);
    print defined($parser->startrule($_)) ? "Good!\n" : "Bad!\n";
}

__DATA__
a a a
a a
[download]

Now the result is "Good!" for both 'a a a' and 'a a' as desired

Hope this helps, -gjb-