use strict;
use warnings;
use Tk;

my $mw = new MainWindow;
$mw->geometry("420x150");

my $input_frame = $mw->Frame()->pack(-fill => 'x');
my $output_frame = $mw->Frame()->pack(-fill => 'x');
my $process_frame = $mw->Frame()->pack(-fill => 'x');

$input_frame->Label(-text => '   Input:   ')->pack(-side => 'left');
my $inputfile_entry = $input_frame->Entry(-width => 50)->pack(-side => 'left');
$input_frame->Button(-text => ' suchen    ',-command => [\&getFile, 'INPUT', $mw, $inputfile_entry])->pack(-side => 'left');

$output_frame->Label(-text => '   Output:')->pack(-side => 'left');
my $outputfile_entry = $output_frame->Entry(-width => 50)->pack(-side => 'left');
$output_frame->Button(-text => '  definieren',-command => [\&getFile, 'OUTPUT', $mw, $outputfile_entry])->pack(-side => 'left');


my $listbox = $process_frame->Listbox(-height => 200)->pack(-fill  => 'both');
my $scroll = $listbox->Scrollbar(-command => [ 'yview', $listbox ]);
$listbox->configure(-yscrollcommand => [ 'set', $scroll ]);$scroll->pack(-side => 'right',              -fill => 'y',             );
$process_frame->Button(-text => 'Zeilennummerierung hinzufügen',-command => [\&addLinenumbers, $mw, $inputfile_entry, $listbox])->pack(-side => 'left');
$process_frame->Button(-text => 'Schreibe Ausgabedatei',-command => [\&writeFile, $mw, $outputfile_entry, $listbox])->pack(-side => 'left');
$process_frame->Button(-text => 'Script beenden',-command => sub { exit })->pack(-side => 'left');

MainLoop();


my @types = (["Textdatei", ['.txt']]);


sub getFile
        {
        my($action, $datAuswahl, $entry) = @_;
        my($filename)="";
        #my $filetypes = [ ['Textfiles', ['.txt'] ] ];
        if($action eq 'INPUT')
                {
                $filename = $datAuswahl->getOpenFile(-filetypes => \@types, -defaultextension => '.txt',-title => 'Select inputfile...');
        
        
        
        
        }
        elsif($action eq 'OUTPUT')      
                {
                $filename = $datAuswahl->getSaveFile(-filetypes => \@types, -defaultextension => '.txt',-title => 'Select outputfile...');
                }
        else    
                {       
                warn "getFile(), no action specified!\n";
                }
        if($filename ne '') 
                {
                $entry->delete(0, 'end');
                $entry->insert(0, $filename);
                }
        }

....