...
my $change_mode = \&change_mode;
...
#Case 1:  Callback routine called directly with anonymous reference
$main->Button(-text => 'To Lower Case',
              -command => sub { &change_mode(1) }
              )->pack;
...              
#Case 2:  Callback routine called with argument via named reference ver. 1      
$main->Button(-text => 'To Upper Case',
              -command => &{$change_mode}(2)
              )->pack;
#Case 3:  Callback routine called with argument via named reference ver. 2      
$main->Button(-text => 'To Mixed Case',
              -command => $change_mode->(3)
              )->pack;