It works in 5.8, althougth the Tk version is probably more important. I can't see why the button wouldn't work for you. I changed the code to count up with each click.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $test = shift || 0;

my $mw = MainWindow->new;
$mw->withdraw;

my $count = 0;

$mw->Button(
    -text    => "hello",
    -command => sub { print $count++,"\n";  }
)->pack;

if ( $test == 0 ) {
    my $answer = $mw->messageBox(
        -icon    => 'question',
        -message => 'Hello World!',
        -title   => 'My title',
        -type    => 'Ok'
    );
    $mw->deiconify;
}
else { $mw->deiconify; }

MainLoop;
[download]