my $ParseError;

BEGIN {
    open(my $olderr, '>&', STDERR) or die "Cannot dup STDERR: $!";
    close STDERR;
    open(STDERR, '>', \$ParseError) or die "Cannot open in-memory file: $!";
    select STDERR; $| = 1;

    # Cannot use, since that messes up execution order for some reason (since use 
    # implies a BEGIN block itself?).
    require Parse::RecDescent;

    close STDERR;
    open(STDERR, '>&', $olderr) or die "Cannot restore STDERR: $!";
}

# Then later:

my $p = Parse::RecDescent->new($grammar) or die "Invalid grammar";
my $output = $p->startrule($str) or die $ParseError;

##</code><code>##

    open(STDERR, '>', '/tmp/error') or die $!;