Tk has limitations, but not this one;-).

In this case, you have to pass the widget into your sub. For example:

#!/usr/bin/perl
use Tk;
use strict;
use constant BUTTON_WIDTH => 20;

my $mw = new MainWindow(title => "demo");
my $button = $mw->Button(text => "color", 
                         width => BUTTON_WIDTH)
                ->pack;
$mw->Button(text => "red",
            width => BUTTON_WIDTH,
            command => sub {change_color($button, "red")})
   ->pack;
$mw->Button(text => "green",
            width => BUTTON_WIDTH,
            command => sub {change_color($button, "green")})
   ->pack;
MainLoop;

sub change_color {
    my ($widget, $color) = @_;
    $widget->configure(background => $color);
}
[download]