About the use of require in this example... Here is a complete working example that captures Parse::RecDescent output in a dup'd anonymous filehandle, which shows that use and require are not equivalent in this case.

Anyway, it would be cleaner to re-open Parse::RecDescent::ERROR.

Yes, it definitely would be. This is what I tried first. Here is a complete example (did you try to run your own example code?):

use strict;

use Parse::RecDescent;

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

    open(local *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 $error;

    return $x;
}

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

$ perl recdescent3.pl 
Undefined format "Parse::RecDescent::ERROR" called at /usr/share/perl5
+/Parse/RecDescent.pm line 2910.
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Clearly format has a side-effect that prevents the use of the nice solution this time. Besides that, your re-open of ERROR requires knowledge of the package internals, while redirecting STDERR requires arguably less knowledge, and certainly not the name of a private (albeit package global) variable.

Regardless of what the documentation says about use being equivalent to require Module; import Module;, this example shows that it is not the case this time.

Not true. The documentation doesn't say that. Both the documentation and I said use Module; is equivalent to

BEGIN { require Module; import Module; }
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And that is clearly the case this time.

Besides that, your re-open of ERROR requires knowledge of the package internals

So does knowing STDERR is duped at execution time. Besides, the benefits far outweigh the drawbacks. Well, if it had worked.

Benefits

• Works with scalar handles.
• Much cleaner code in the caller.
• In fact, one can arrange to have no extra code in the caller since it can be placed in the "grammar".
• Works even if use Parse::RecDescent; is executed twice.
• Works with threads.

Drawbacks

• Minor reliance on stable PRD guts.
• Doesn't work. Oops!

Clearly format has a side-effect that prevents the use of the nice solution this time.

Ah dang! You could issue the format on the new handle, but that's going pretty far into the inards.

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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The reason local breaks it is that local replaces the variable in that lexical scope, and the references to P::RD::ERROR in P/RD.pm are outside the lexical scope of the local invocation (I think).

It's an interesting question though, and IMO the right solution would be to get a patch committed for P::RD that allows the user to specify an alternative filehandle for error output.