Curious; as it happens I had to do this for a project I was working on some months back, and came to the same conclusion as ikegami. My code reads thusly (copied and pasted):

    open( *Parse::RecDescent::ERROR, '>', \(my $parse_error) )
    or croak("Error: unable to redirect SDTERR.");

    $Parse::RecDescent::skip = ' *\x{0} *';

    $::RD_ERRORS++;
    $::RD_WARN++;
    $::RD_HINT++;

    my $parser = Parse::RecDescent->new($grammar) or die("Bad grammar!
+");
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I can assure you that does actually work for me (perl 5.8.8, P::RD 1.94). My best guess is that your use of local on that open call is maybe creating problems?

The dependency on P::RD internals always bothered me as well, but I never got around to figuring out a better way to do this.

Cheers, Tim

Update: Yup, turns out if you remove that local then it works:

use strict;

use Parse::RecDescent;

sub parse {
    my ($grammar, $str) = @_;

    open(*Parse::RecDescent::ERROR, '>', \my $error) 
        or die "Cannot open in-memory filehandle: $!";

    local $::RD_ERROR = 1;
    local $::RD_WARN = 2;

    my $p = Parse::RecDescent->new($grammar)
        or die "Grammar is invalid";

    my $x = $p->start($str);
    defined $x or die "CAPTURED: $error";

    return $x;
}

print parse('start: /foo/ | <error>', 'fo'), "\n";
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