I had the same issue today and found this thread. Yes, it is old, but I would like to add another solution in case someone needs it.

You can use a timer to wait for a moment before you do the resize. Cancel the timer if the MainWindow resizes again within a certain time and start it anew.

Sample code:

#!perl

use strict;
use warnings;
use Tk;
use v5.16;

# Skript zum manuellen resizen der Canvas nach Änderung der Größe des 
+MainWindow.
# Man muss nochmal einen Rechtsklick machen.
# Quelle: https://www.perlmonks.org/bare/?node_id=581309, abgerufen 20
+20-11-21


my $mw = Tk::MainWindow->new;

my $current_size = $mw->reqwidth . "x" . $mw->reqheight;
my $old_current_size = $current_size;

my $canvas = $mw->Canvas(
    -width => 300,
    -height => 300,
    -bg => 'black',
)->pack();

$canvas->update; # important to call this before binding <configure> e
+vent

my $after_resize_timer_id;
$mw->bind('<Configure>' => sub {
    say "MW resized";
    
    # reset timer again, if it was set
    $mw->afterCancel($after_resize_timer_id) if defined $after_resize_
+timer_id;
    
    # set timer
    # if MW doesn't resize again within a second or so, resize Canvas
    # if it does, reset timer to wait for a second or so
    $after_resize_timer_id = $mw->after(500, sub{
        say "Update canvas";
        my $width = $mw->width;
        my $height = $mw->height;
        $canvas->configure(-width => $width, -height => $height);
        $canvas->update;
    });
    
});

$mw->MainLoop;
exit(0);
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