I don't think that syntax is valid. Even if "true" and "false" were declared as constants that would be something like $image->Append(stack => { 1 => 0}) I think you really want to just do:

$x = $image->Append(stack=>0);
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Update: After testing, I found you need to explicitly set stack to 0. Despite what the docs say, Image::Magick 6.4.8 seems to stack by Default.