I think that your style of code is good for maintainability.

I like the use constant approach that keszler mentioned. There is a module that can do the counting for you; it is called enum::fields. You also have the opportunity to remove some redundancy from your array initializations.

sub guiStart
{
  my ($client,$login) = @_;

  my $mw = MainWindow->new(-borderwidth => 30,
                           -title       => "Duplicates: ".$client);

  use enum::fields qw{ LIST DEL ADD DUP CHG EXIT };

  my %layout;
  $layout{label} = [
                     'List Available Data',
                     'Remove Data',
                     'Add Data',
                     'Identify Duplicates',
                     'Change Client',
                     'Exit Application',
                   ];

  $layout{callback} = [
                         \&listData,
                         \&removeData,
                         [\&addData,,$client,$login,$mw],
                         \&identifyDuplicates,
                         \&changeClient,
                         \&exitApp,
                      ];

  for my $btn (LIST..EXIT)
  {
    $layout{frame}->[$btn]  = $mw->Frame();
    $layout{button}->[$btn] = $layout{frame}->[$btn]
                      ->Button(-text    => $layout{label}->[$btn],
                               -width   => 20,
                               -command => $layout{callback}->[$btn])
                      ->pack();
    $layout{frame}->[$btn]->pack();
  }
  MainLoop;
  return;
}
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