my $button = $mw->Button(-text => $label);

##</code><code>##

    $button->configure(-command => [ \&show_button_label, $button ]);

    -or-

    $button->configure(-command => sub { show_button_label($button) });

##</code><code>##

#!/usr/bin/perl

# Libraries
use strict;
use warnings;
use Tk;

# User-defined
my @button_labels = (qw(
    Red Blue Green White Yellow Orange
));

# Main program
my $mw = new MainWindow();
foreach my $label (@button_labels) {
    # Note labels are colors, so they can be used for the background
    my $button = $mw->Button(-text => $label, -bg => $label);
    $button->configure(-command => [ \&show_button_label, $button ]);
    $button->pack(-side => "left");
}
MainLoop();


# Subroutines
sub show_button_label {
    my ($button) = @_;
    my $label = $button->cget(-text);
    print "Label for button '$button' is '$label'\n";
}