It's true that you can use OnDestroy to invoke some code when a widget is destroyed, but only if it's not the MainWindow.  If it's the MainWindow that's destroyed, it's too late, as you've discovered.

Here's a way to test it.  With some slight modifications to your code and the addition of a boolean variable $b_destroy_button you can try running the following:

use strict;
use warnings;
use Tk;
use Tk::DialogBox;

# User-defined
my $b_destroy_button = 0;  # If set, destroy the Button, not the MainW
+indow

# Main program
my $mw = MainWindow->new();
my $button = $mw->Button(-text => 'Exit');

# Choose which widget to destroy
my $psub;
($b_destroy_button)  or $psub = sub { $mw->destroy() };
($b_destroy_button) and $psub = sub { $button->destroy() };

$button->configure(-command => $psub);
$button->pack(-side => 'right', -ipadx => 10, -padx => 30);
$button->OnDestroy(\&exit_app);
MainLoop;

# Subroutines
sub exit_app{
    print "1\n";
    my $db = $mw->DialogBox(-title => 'Error',-buttons => ['CSV','Aban
+don']);
    my $text = "Do you wish to save a new CSV file or abandon changes?
+";
    $db->Label(-text => $text)->pack();
    my $button = $db->Show();
    if ($button eq 'CSV'){
        save_csv();
    }
}

sub save_csv{
    return 1;
}
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Now if you run it "as is", you'll get the same behavior, because destroying the MainWindow causes a problem.

But when you destroy just the Button instead, by changing the value of $b_destroy_button to 1, it will successfully give you the DialogBox you want.

An alternative way to do this (if you don't want to have to destroy a widget before taking the action of possibly writing the CSV file), is to just put the call to exit_app() in the callback for the Button.  Then, after you're done writing the CSV file (or not), you can have the callback destroy the application, like so:

use strict;
use warnings;
use Tk;
use Tk::DialogBox;

# Main program
my $mw = MainWindow->new();
my $button = $mw->Button(-text => 'Exit');
$button->configure(-command => \&exit_app);
$button->pack(-side => 'right', -ipadx => 10, -padx => 30);
MainLoop;

# Subroutines
sub exit_app{
    print "1\n";
    my $db = $mw->DialogBox(-title => 'Error',-buttons => ['CSV','Aban
+don']);
    my $text = "Do you wish to save a new CSV file or abandon changes?
+";
    $db->Label(-text => $text)->pack();
    my $button = $db->Show();
    if ($button eq 'CSV'){
        save_csv();
    }
    $mw->destroy();
}

sub save_csv{
    return 1;
}
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I had tried the latter example. The problem is that the exit_app() sub gets bypassed if you shut down using the 'X' button rather than the 'Exit' button.

I have come up with

use strict;
use warnings;
use Tk;
use Tk::DialogBox;

my $exit_pressed = 0;

my $mw = MainWindow->new();
$mw->OnDestroy(\&exit_x);

$mw->Button(-text => 'Exit', -command=> \&exit_app)->pack(-side=>'righ
+t', -ipadx => 10, -padx=>30);


MainLoop;

sub exit_app{
    print "1";
        $exit_pressed = 1;
        my $db = $mw->DialogBox(-title => 'Save?',-buttons => ['CSV','
+Abandon']);
        $db->Label(-text => "Do you wish to save a new CSV file or aba
+ndon changes?")->pack();
        my $button = $db->Show();
    if ($button eq 'CSV'){
        save_csv();
        }
        $mw->destroy();
}

sub save_csv{
    return 1;
}

sub exit_x{
    unless ($exit_pressed){

        print "2";
            my $check = MainWindow->new(-title=> 'Save?');
            $check->Label(-text => "Do you wish to save a new CSV file
+ or abandon changes?")->pack();
            $check->Button(-text => 'CSV', -command => sub { save_csv(
+);$check->destroy();})->pack(-side=>'bottom');
            $check->Button(-text => 'Abandon', -command => sub {$check
+->destroy();})->pack(-side=>'bottom');
        }
}
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However it a) isn't elegant from a code point of view, b) means that the screen (which will contain data) is closed before the option to have it is displayed and c)It will be hard to make the 2 save windows look the same.

Solved it.

Replace $mw->Destroy(\&exit_app) with $mw->protocol('WM_DELETE_WINDOW' => \&exit_app)

use strict;
use warnings;
use Tk;
use Tk::DialogBox;

my $mw = MainWindow->new();

my $button = $mw->Button(-text => 'Exit', -command=> \&exit_app)->pack
+(-side=>'right', -ipadx => 10, -padx=>30);

$mw->protocol('WM_DELETE_WINDOW' => \&exit_app)  ;

MainLoop;

sub exit_app{
    print "1";
        my $db = $mw->DialogBox(-title => 'Save?',-buttons => ['CSV','
+Abandon']);
        $db->Label(-text => "Do you wish to save a new CSV file or aba
+ndon changes?")->pack();
        my $button = $db->Show();
    if ($button eq 'CSV'){
        save_csv();
        }
        $mw->destroy();
}

sub save_csv{
    return 1;
}
[download]