There are 3 things you need to consider. First, the scrollregion will define how big the real Canvas actually is. Second, the -width and -height will specify how big the Scrolled window will be. ( Assuming the window it is packed in will allow it). Without a $mw->geometry(), the $mw will fill itself according to the widgets packed in it.

If you comment out the plus button, as below, you will see this. Placing a button directly into the canvas widget is wrong, you use a window. When you place it onto the canvas you get that weird sizing behavior. Don't do that. :-)

#!/usr/bin/perl -w             
use Tk;
use strict;  
                                                
my $mw = MainWindow->new;

#$mw->geometry('50x50'); #try this
 
my $c = $mw->Scrolled('Canvas',
-width => 200,
-height => 200,
-background =>'blue',
-scrollregion => [ 0, 0, 500, 500 ]
)->pack(-expand => -2, -fill => 'both');

$c->createRectangle(100, 100, 150, 150, -fill => 'yellow');

#my $plus = $c->Button( 
#   -text    => '  +  ', 
#       -command => sub {$c->scale("all", 0, 0, 2, 2); } );
#   $plus->pack;
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