On rereading your post and responses, I think it probably is Tk::Entry that you're talking about.

There's a few tricky bits and gotchas when setting up -state => 'readonly'; and also when button presses result in writing to a readonly widget. So, here's some code to get you started.

#!/usr/bin/env perl

use strict;
use warnings;

use Tk;

my $mw = MainWindow::->new(-title => "Ken's Guess");

my %frame_pack_opts = qw{-fill x -padx 10 -pady 10};
my $app_preamble_F = $mw->Frame()->pack(%frame_pack_opts);
my $letter_buttons_F = $mw->Frame()->pack(%frame_pack_opts);
my $copy_from_F = $mw->Frame()->pack(%frame_pack_opts);
my $paste_to_F = $mw->Frame()->pack(%frame_pack_opts);
my $exit_F = $mw->Frame()->pack(%frame_pack_opts);

chomp(my $app_preamble = <<'EOT');
The "Copy from:" entry can only be modified with the
"Select:" buttons.
Any text here may be selected and copied.

The "Paste to:" entry can be modified in any way you
want. It's primary function was to test that text
copied from the "Copy from:" entry could be pasted;
however, you can also type text from the keyboard,
select existing text, and delete text.
EOT
$app_preamble_F->Label(-text => $app_preamble, -justify => 'left'
)->pack(-side => 'left');

my $copy_from_entry;
my $copy_from_value = '';

$letter_buttons_F->Label(-text => 'Select: ')->pack(-side => 'left');
for my $letter ('A' .. 'G') {
    $letter_buttons_F->Button(-text => $letter,
        -command => sub {
            $copy_from_entry->configure(-state => 'normal');
            $copy_from_value = $letter;
            $copy_from_entry->configure(-state => 'readonly');
        }
    )->pack(-side => 'left');
}

$copy_from_F->Label(-text => 'Copy from: ')->pack(-side => 'left');
$copy_from_entry = $copy_from_F->Entry(
    -textvariable => \$copy_from_value, -state => 'readonly'
)->pack(-side => 'left');

chomp(my $paste_to_preamble = <<'EOT');
I have to assume that the widget, where you want
to paste this text, is part of another application.
Otherwise, this makes no sense at all.
EOT
$paste_to_F->Label(-text => $paste_to_preamble, -justify => 'left'
)->pack(-anchor => 'w', -pady => 5);
$paste_to_F->Label(-text => 'Paste to: ')->pack(-side => 'left');
$paste_to_F->Entry()->pack(-side => 'left');

$exit_F->Button(-text => 'Exit', -command => sub { exit; })->pack();

MainLoop;
[download]

This code represents a complete, working GUI. I suggest running it and understanding how it works; then make a copy and adapt that for your needs.

I'm still somewhat mystified with regards to what your ultimate goal might be. Have a read of "XY Problem" and ask yourself if that might apply here.

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