I think you are going to run into problems with this type of script, where you combine Tk and GD.

Hmm. Provided that you disard old things before creating new ones, the space should be reused.

I haven't put this in a tight loop and run it thousands of times, but I've clicked the button quite a lot (maybe a 100 times) whilst monitoring the memory, and I see no sign of a growth trend (on my system):

use Tk;
use Tk::PNG;
use MIME::Base64;

use GD::Graph;
use GD::Graph::bars;

my $data = [
   [qw/1st 2nd 3rd 4th 5th 6th 7th 8th 9th/],
   [qw/  1   2   5   6   3 1.5   1   3   4/],
];

my $graph = GD::Graph::bars->new(400, 300);
$graph->set(
   x_label        => 'X Label',
   y_label        => 'Y label',
   title          => 'Some simple graph',
   y_max_value    => 8,
   y_tick_number  => 8,
   y_label_skip   => 2
) or die $my_graph->error;

my $gd = $graph->plot($data) or die $my_graph->error;

my $mw = MainWindow->new;
my $png = $mw->Photo(-data => encode_base64($gd->png));

$mw->Button( -text => 'doit', -command => sub{
    $data->[ 1 ] = [ map{ rand 5 } 1 .. 9 ];
    undef $graph;
    $graph = GD::Graph::bars->new(400, 300);
    $graph->set(
       x_label        => 'X Label',
       y_label        => 'Y label',
       title          => 'Some simple graph',
       y_max_value    => 8,
       y_tick_number  => 8,
       y_label_skip   => 2
    ) or die $my_graph->error;

    my $gd = $graph->plot($data) or die $my_graph->error;
    $png->blank;
    $png->configure( -data  => encode_base64($gd->png) );
    $mw->update;
} )->pack;

$mw->Label(-image => $png)->pack;

MainLoop;
[download]