G'day Kaughper,

Welcome to the Monastery.

"There is no code in the button command that even mentions the items for the first Canvas."

Well, actually there is. You've given $imgA the name 'image'. In &Add_B, you've used the same Photo named 'image'.

In addition, that's potentially confused with the name of the Canvas item type:

$cvsA->create('image', ...);
$cvsB->create('image', ...);
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A quick fix would be to name them 'imageA' and 'imageB'. I've tested that and it works.

You have a more insidious problem waiting to happen: &Add_B relies on a variable not passed to that subroutine. This is one of the most common problems with Tk programs; fortunately, it is easily avoided. Consider changing the code after defining your XPMs to something like this (which I've successfully tested):

my %image_data_for = (
    A => {
        name => 'ImageA',
        data => $LtrA,
    },
    B => {
        name => 'ImageB',
        data => $LtrB,
    },
); 

# Image A holder in Main Window
$mw->Label(-text => 'Image A:')->pack();
my $cvsA = $mw->Canvas(-background=>'yellow', -height => 40, -width =>
+ 40)->pack();

# Image B holder in Main Window
$mw->Label(-text => 'Image B:')->pack();
my $cvsB = $mw->Canvas(-background=>'green', -height => 40, -width => 
+40)->pack();

# Set Image A
create_canvas_image($cvsA, $image_data_for{A});

# Button to make Image B
$mw->Button(
    -text => 'Add Letter B',
    -command => [\&create_canvas_image, $cvsB, $image_data_for{B}],
)->pack(-pady => 5);

MainLoop;

sub create_canvas_image {
    my ($canvas, $image_data) = @_;

    my $img = $mw->Photo($image_data->{name}, -data => $image_data->{d
+ata});
    $canvas->create('image', 30, 30, -image => $img);
}
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Now you only have one routine (&create_canvas_image) to create all images: your example, which I'm guessing is just a proof-of-concept, only has two; but this will work for whatever number of images you want in your real-world application.

Please note that I've only shown an example for demonstrating a technique. For production-grade code, I'd be limiting the scope of most of the variables: giving them all file-scope is not a good idea as any subsequent code after their definition can change their values.