G'day jeri_rl,

Welcome to the monastery.

Just to clarify, if you have something like "... -command => sub { ... } ..." in your code, the "sub { ... }" part is the callback and it cannot return anything to the main program. However, code within the callback can return values. Here's another version of my original script that does just that:

#!/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 { $tissue = sele
+ct_tissue() })->pack;
$f1->Button(-text => 'Quit', -command => sub { do_end($tissue) })->pac
+k;

MainLoop;

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

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

If you run this code, you'll see it works identically to my original script. Of course, this is a very simple GUI with very straightforward subroutines. With more complex code, you can run into problems when passing values instead of references; on occasion, these can be hard to track down and rectifying them can involve reworking large parts of your code. I'd recommend references over values in this scenario. There's an added benefit of references being scalars, so you're potentially moving a lot less data around than you would with, say, @array_with_lots_of_elements or %complex_data_structure.