In case it helps speed things up for you, here's a minimalist version of your code that allows for the grammar to be typed/pasted into a separate text box, so you can modify the grammar on the fly without having to re-edit and re-start the script:

#!/usr/bin/perl

use strict;
use Tk;
use Parse::RecDescent;

my $mw = new MainWindow;
my $frm = $mw->Frame()->pack();
my $mw1 = $frm->Text()->pack(-side => 'left');
my $mw2 = $frm->Text()->pack(-side => 'left');
my $mw3 = $mw->Button(-text=>"Check brackets",
                      -command => [ \&parser, $mw1, $mw2 ]
                      )->pack();
MainLoop;

sub parser {
    my ($grm,$txt) = @_;
    my $grammar = $grm->get('1.0','end');
    my $parser = Parse::RecDescent->new($grammar);
    if ( !defined( $parser )) {
        warn "BAD GRAMMAR\n";
        return;
    }
    if ( defined( $parser->startrule( $txt->get( "1.0", "end" )))) {
        print "NO ERROR\n";
    } else {
        print "TEXT COULD NOT BE PARSED\n";
    }
}
[download]

As you play with that, you'll notice that Parse::RecDescent gives informative messages about problems with the grammar when it's not usable (but you probably know that already).