Tried it on perl 5.8.4-810 and same problem.
For some strange reason this works:

    use strict;
    use Tk;
    my $mw = MainWindow->new;   
    my $t = $mw->Scrolled('Text',-scrollbars => 'se', -wrap=>'none')->
+pack;
        #Works
    $t->Subwidget('text')->configure(-bg => 'grey');
        #Does not work
    #$t->Subwidget('yscrollbar')->configure(-width =>24);
     
    $mw->after(100, sub {$t->Subwidget('yscrollbar')->configure(-width
+ =>44);$mw->update;});
    MainLoop;
[download]

Can't go any lower than 100ms on the timer, then it doesn't work..therefore the resize of the scrollbar is clearly visible in the mainwindow..

annoying..Can i spit?

Update:Ok..seems like timer be lowered down to 40ms..which makes change less visible..