Unfortunately, (or fortunately :-) the widgets are not of consistent size and so -width does not look like it will be useful within -grid. (One other twist is that I have 'styles' of layout, such that the button's appearance varies from style to style.)

I do see that in configuring button $Output_b (and to handle multiple styles) I can do:

    -height => $style eq 'workstation'
            ? 6
            : 3,  # style eq something else...
[download]

but now every other button on the same row as $Output_b has a minimum height of 3 text-rows (oops) and instead of fooling around with $Output_b's options I want to target all my adjustments within the -grid sections. I suppose I could throw in extra button configures as necessary within my -grid/style statement blocks, but ideally I shouldn't be distracted with altering other buttons as I configure the $Output_b button.

Instead I have gone ahead with configuring dummy cells (blank Label widgits) within the 'style' that needs it, i.e.:

my $cell_of_spaces = ' ' x 12;

## I've already set up the $Option_b button and am in the
## midst of configuring a "workstation" style/layout

$Output_b->grid( 
    -column => 5,    
    -row => 9,    
    -columnspan => 4,    
    -rowspan => 6,    
    -sticky => 'nsew',
);


my $space_cell_col = 9;
    # column 9 already acted as a separator between two big
    # buttons, so no problem putting blank Labels here.

foreach my $space_cell_row (9 .. 14) {
    build_blank_cell(
        $tl,
        $cell_of_spaces,
        $space_cell_col,
        $space_cell_row,
            );
}


# Now that there are cells on each row between 9 and 14,
# the $Output_b button's rowspan option takes full effect.

sub build_blank_cell {
    my (
        $tl,
        $cell_of_spaces,
        $space_cell_col,
        $space_cell_row
            ) = @_;

    my $new_label_widget_l = $tl->Label(
        -textvariable => \$cell_of_spaces,
            )->grid( 
                -column => $space_cell_col,
                -row => $space_cell_row,
                -sticky => 'nsew',
                    );
}
[download]