First of all, use strict; will help you to find some variable declaration errors !

The big problem is, that you wrote TopLevel instead of Toplevel
This is a quick shoot for your script:

use strict;
use warnings;
use Tk;

my $listbox;
my $mw=MainWindow->new;
$mw->geometry('800x600');

my $menu_bar = $mw->Frame();
my $rf = $mw->Frame;


my $search_mb = $menu_bar->Menubutton('-text'         => 'File',
                                   '-relief'       => 'raised',
                                   '-borderwidth'  => 2,
                                   )->pack('-side' => 'left',
                                           '-padx' => 2
                                           );



$search_mb->command('-label'       => 'Open',
                    '-accelerator' => 'Ctrl+o',
                    '-underline'   => 0,
                    '-command'     => \&open_file
                    );

$search_mb->command('-label'       => 'Save',
                    '-accelerator' => 'Ctrl+s',
                    '-underline'   => 0,
                    '-command'     => \&save_file
            );

$search_mb->command('-label'       => 'Close',
                    '-accelerator' => 'Ctrl+x',
                    '-underline'   => 0,
                    '-command'     => \&close_file
            );
$search_mb->command('-label'       => 'Exit',
                    '-accelerator' => 'Ctrl+q',
                    '-underline'   => 0,
                    '-command'     => [$mw => 'destroy']
                    );

my($InputText) = $rf->Scrolled('TextUndo',
                   -height => '1',
                   -width => '1',
                   -scrollbars => 'osoe',
                   );




$menu_bar->pack(-anchor => 'nw');
$rf->pack(qw/-side right -fill both -expand 1/);
$InputText->pack(qw/-side top -fill both -expand 1/);

MainLoop;



sub open_file{

my $open = $mw->Toplevel;
$open->geometry('400x550');
my $tf = $open->Frame;
my $bf = $open->Frame;

$listbox = $tf->Listbox(-height => '10',
            -selectmode => 'browse',
                -width => '40',
                -relief => 'sunken',
                        -setgrid => '1');


$listbox->delete('0.1', 'end');


opendir DIR, ".";
$listbox->insert('end', grep { -f $_ && -r $_ } readdir DIR);
close DIR;

my $button = $bf->Button( -command => \&load,
            -text => 'Open');



$tf->pack;
$bf->pack;
$button->pack(-anchor => 'se');
}

sub load{

    my ($index) = $listbox->curselection();


    my $filename = $listbox->get($index);


    $InputText->Load( $filename  );


    (my $script = $0) =~ s,.*(\/|\\),,;

}
[download]

As you can see, you just have to use Tk and all the other required modules need not to be used here.

Hope this helps.

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