use strict;
use warnings;
use Tk;
my $widgets = {};
$widgets->{main} = MainWindow->new;
$widgets->{button} = $widgets->{main}->Button(
    -text => "Whatever you do - don't push me",
    -command => [ \&callme, $widgets ]
)->pack;
MainLoop;

sub callme
{
    my $w = shift;
    $w->{main}->configure(-title => 'How dare you push me!');
    # Anything else in the $widgets hash is available here
    # too - handy if you have lots of widgets you need to
    # control from your functions or methods.
}