There is a workaround on described on http://www.rhinocerus.net/forum/lang-perl-tk/187668-checkbutton-problem.html which I also implemented in the following code. This workaround disables a click functionality for a button before calling getOpenFile and restores it after little delay. Is there a wise monk who knows more elegant solution?

#!/usr/local/bin/perl -w

use strict;
use warnings;
use Tk;
use utf8;
use v5.12;




my $mw = Tk::MainWindow->new(
        -title => 'main window',
        );
        
        
#Exit when the escape key is pressed
$mw -> bind('<Key-Escape>', sub { exit });

my $frame = $mw->Frame(-borderwidth => 3)->pack( -fill => 'both',);

my $btn1 = $frame->Button(
        -text => 'button',
        -command => sub { buttonClicked() },
        -width => 120,
        -height => 40,        
    )->pack(-fill => 'both',);        

$frame->Button(
        -text => 'file selector',
        -command => sub { select_file() },
    )->pack(-fill => 'both',);    
$frame->Button(
        -text => 'exit',
        -command => sub { exit },   
    )->pack(-fill => 'both',);    
        
MainLoop;


sub select_file {
    breakCheckbuttonBindings();
    my $file = $mw->getOpenFile(
        -title => 'select file',
        );
    $file = File::Spec->canonpath( $file );
    $mw->after(25,\&restoreCheckbuttonBindings);
    return $file;
}

sub buttonClicked {
        $mw->messageBox(-message=>"you clicked a button!");
}


sub breakCheckbuttonBindings {
    my @tags = $btn1->bindtags;
    $btn1->bindtags([@tags[1,0,2,3]]);
    $btn1->bind('<ButtonPress>'=>sub{Tk->break});
}

sub restoreCheckbuttonBindings {
    my @tags = $btn1->bindtags;
    $btn1->bindtags([@tags[1,0,2,3]]);
    $btn1->bind('<ButtonPress>'=>'');
}
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