#!/usr/bin/perl --
use strict; use warnings;
use Tk;

my $packArgs = do {
    my @packargs = (
        [ qw/-expand 0 / ],
        [ qw/-expand 1 -fill x / ],
        [ qw/-expand 1 -fill y / ],
        [ qw/-expand 1 -fill both / ],
    );
    my $ix = -1;
    my $xx = @packargs ;
    sub {
        $ix++;
        warn $ix % $xx ;
        return @{ $packargs[ $ix % $xx  ] };
    };
};

my $mw = tkinit;
$mw->Button(
    -text => 'BANG!',
    -command => \&bangBang
)->pack(qw/-side top -anchor n/);

my $ca = $mw->Canvas( background => 'white' ) ->pack( $packArgs->() );
$ca->createOval(
    50, 50,
    100, 100,
    -fill => 'pink',
);

MainLoop;
exit 0;
sub bangBang {
    my $b = $Tk::event->W;
    
    $mw->Busy;
    $b->configure(-state => "disabled");
    
    my $text = $b->cget(qw/ -text /);
    unless( $text =~ s/(\d+)/ $1 + 1 /e ){
        $text .= " 1";
    }
    $b->configure( -text => $text);
    $ca->packForget;
    $mw->update;
    sleep 1;
    $ca->pack( $packArgs->() );
    
    $mw->Unbusy;
    $b->configure(-state => "normal");
    $mw->update;
}