Maybe something like:

use strict;
use warnings;

use Tk;

my $MW = tkinit;

my $show_button_text = 'txt';
my $show_button = $MW->Button(
    -textvariable => \$show_button_text,
    -command      => sub { print "$show_button_text \n"; },
    )->pack(qw(-side top  -fill x));

my %add_buttons;
for my $add_text (qw(a bb xyz)) {
    $add_buttons{$add_text} = $MW->Button(
        -text    => $add_text,
        -command => sub { $show_button_text .= " $add_text"; },
        )->pack(qw(-side left  -expand 1));
    }

MainLoop;
[download]

Maybe you want to store it in the clipboard directly, using Tk::clipboard, or maybe Clipboard?