G'day James,

++ It worked as described for me.

There's an unlimited amount of times that "next" can be used. You may want to add a limit. At the moment, for the first several uses, the proportions are retained; something like this:

+-----+
|     |
|     |
|  O  |
|     |
|  O  |
|     |
|     |
+-----+

After half a dozen uses, the layout degrades; very roughly like this:

+-----+  +-----+  +-----+  +-----+
|     |  |     |  |     |  |     |
|     |  |     |  |     |  |     |
|  O  |  |  O  |  |  O  |  |  O  |
|     |  |     |  |     |  |     |
|  O  |  |  O  |  |  O  |  +-----+
|     |  |     |  +-----+
|     |  +-----+
+-----+

As a general rule-of-thumb in Tk applications, unless I want to pass an absolute value to a callback which doesn't change that value, I use references. In many cases, even if the value isn't being changed in the callback, it may be changed prior to the callback being invoked; references are typically a good option in these cases also.

You also may want add a "prev" button to test down-scaling.

Not intended as a huge criticism, but I do see the following code repeated:

$canvas->createPolygon(@scaledBox,   -fill => 'darkgreen');
$canvas->createOval(@scaledTopColon, -fill => 'yellow');
$canvas->createOval(@scaledBotColon, -fill => 'yellow');
[download]

Consider abstracting that into a single routine. If you subsequently want to change something, you'll only need to do it in one place (see "DRY principle").