The docs of "Read or Write an Image" (in the debian packaged 6.6.0.4) describe %d expanding to the image number in Write(). If I'm not mistaken that happens to all writes, not just a multi-image. Is there a way to say that the filename=> parameter is to be taken literally? Or a way to escape a % form like that in the filename?
I don't imagine a %d is very likely in something a user would enter, but in some code working with single images I thought I might prefer to have all filenames taken literally to avoid surprises.
I suppose this is a general magick thing rather than perl specific. The perlmagick docs might helpfully cross-reference to any further info on the expanding though, if there is some.