In order to do this you need to pass around a reference. Here's a working (albeit barebones) script to demonstrate this:

#!/usr/bin/env perl

use strict;
use warnings;

use Tk;

my $tissue = 'plasma';

my $mw = MainWindow->new;

my $f1 = $mw->Frame()->pack;
$f1->Label(-textvariable => \$tissue)->pack;
$f1->Button(-text => 'Select Tissue', -command => sub { select_tissue(
+\$tissue) })->pack;
$f1->Button(-text => 'Quit', -command => sub { do_end(\$tissue) })->pa
+ck;

MainLoop;

sub select_tissue {
    my $tissue_ref = shift;
    # Tissue selection code here - assume 'muscle' selected
    $$tissue_ref = 'muscle';
    return;
}

sub do_end {
    my $tissue_ref = shift;
    # Print to STDOUT for demo purposes
    print "$$tissue_ref\n";
    exit;
}
[download]

Notes:

• $tissue needs to be declared in a scope that's visible to the -command coderefs.
• The callbacks have a reference to this variable, i.e. \$tissue.
• The callbacks return nothing!
• The value of $tissue is accessed or modified via $$tissue_ref.
• Most widgets that have a -text option also have a -textvariable option. Here I've used a Label - it starts with the default vaue of "plasma", If you click the "Select Tissue" button, the Label text will change to "muscle".
• The output (when you click "Quit") will depend on whether or not the "Select Tissue" button was pressed.

Update: fixed title s/Perk Tk/Perl Tk/

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.