a small nitpick; I would write your click() as:

sub click {
    my $todo = shift;
    system "xmms -$todo";
}
[download]

and change the buttons' callbacks to

$window->Button(-text=>"Prev", -command=> sub{click('r')})->pack(-side
+=>'left');

$window->Button(-text=>"Play", -command=> sub{click('p')})->pack(-side
+=>'left');
$window->Button(-text=>"Pause", -command=> sub{click('u')})->pack(-sid
+e=>'left');
$window->Button(-text=>"Next", -command=> sub{click('f')})->pack(-side
+=>'left');
[download]

You might also want to check wmxmms which sits in some kind of dock and can control xmms for you.