Use the Text widget for multi-line text editing.

use warnings;
use strict;
use Tk;

my $mw = MainWindow->new (-title => "editor");
my $text = $mw->Scrolled
    ('Text', -font => 'normal', -wrap => 'word', -scrollbars => 'e',);
$text->pack ();
$mw->Button (-text => "open file", -command => \&openFile)->pack ();
    
MainLoop ();

sub openFile {
    my @filetypes = (
            ['Text', '.txt',  'TEXT'],
            ['Text', '.txt',  'TEXT'],
        );

    my $currentFile = $text->getOpenFile
        (-defaultextension => '.txt', -filetypes => \@filetypes);
}
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Self modifying code is generally frowned on as being somewhat difficult to maintain. A setting file would generally be the best way to store persistent data, but there are lots of ways of generating and maintaining the settings file. One fairly easy way to do it is ti use Storable; and keep all your context stuff in a hash. Then you can:

use Storable;

# Load persistent stuff
my $persistRef = retrieve ('config') if -e 'config';

# Use stuff
my $text = $persistRef->{text} || '';

# ...
$persistRef->{text} = $text;

# Save stuff
store $persistRef, 'config';
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Update: added file open dialog code to Tk sample

You could use a text field or a few entry fields in a frame.

You could overwrite your program, perhaps the __DATA__ area at the end, but i think this is a bad idea, better store the information somewhere else.

#!/usr/bin/perl
use strict;
use warnings;
use Tk;
use Tk::JComboBox;

my @choices;
my $index = 'IM';

foreach my $f ( 'A' .. 'Z' ) {
    foreach my $s ( 'A' .. 'Z' ) {
        push @choices, "$f$s";
    }
}

print "@choices";

my $mw  = MainWindow->new();

my $jcb = $mw->JComboBox(
    -relief             => 'groove',
#    -popuprelief        => 'groove',
    -mode               => 'editable',
    -validate           => 'match',
    -highlightthickness => 0,
    -choices            => \@choices,
    -textvariable       => \$index,
    -selectcommand      => sub {
                            print "Selected: $index\n";
                           }
)->pack;

MainLoop;
#########################
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Simple Inline Files

#!/usr/bin/perl
use Inline::Files;

print while <PEANUT>;
print while <BUTTER>;

__PEANUT__
Bing
Bong
Bang
__BUTTER__
Foo
Bar
Baz
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And to change a DATA segment

#!/usr/bin/perl
use Inline::Files;
use warnings;
use strict;

#open NUMBER, '+<'; #InlineFiles auto opens 

my $number = <NUMBER>;
seek NUMBER, 0, 0;

print "I am number $number\n";

print NUMBER ++$number;

__NUMBER__
0
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