# generate a closure:
sub gen_change_mode {
    my $mode = shift;
    return {
        # same code as in change_mode

    };
}

# then do this
$main->Button(-text => 'To Upper Case',
    -command => gen_change_mode(2)
    )->pack;

# or this
$some_ref = gen_change_mode(3);

$main->Button(-text => 'To Mixed Case',
    -command => $some_ref
    )->pack;