Well, yet another of my misconceptions (regarding the needfulness of $mw->update) has been brought out into the light of day (where, hopefully, it will wither away and die).

After some experimentation, I think I have something that does what you want. I had to combine several different things together, but that appears to the 'Tk' way. So without further ado...

#!/opt/local/bin/perl -w
use strict;
use warnings;
use Tk;

my $button_label = 'Push me';

my $mw = MainWindow->new;
   $mw->geometry("300x200+20+20");
   $mw->title( "Tk Testing" );

my $button;
$button = $mw->Button(
   -textvariable => \$button_label,
   -underline => 0,
   -foreground => 'blue',
   -command => sub {
       my $saved_label = $button_label;
       $button_label = 'Ouch!';
       $button->configure(
                          -relief => 'flat',
                          -activeforeground => 'red',
                          -activebackground => 'green',
                         );

       $button->Busy(-recurse => 1 );

       sleep( 10 );

       $button_label = $saved_label;
       $button->configure(
                          -relief => 'raised',
                          -foreground => 'blue',
                         );

       $button->Unbusy;
                   },
   )->pack(-side => 'top',
           -anchor => 'center',
           -fill => 'none',
           -expand => 0,
           -padx => 5,
           -pady => 5,
          );

MainLoop;

exit 0;

__END__
[download]

Hope this is useful. Even if it isn't, I've learned something, so it wasn't a complete waste. :)

Pat