What I meant was that you first do "mobi2mobi oldfile.prc --outfile newfile.mobi" and then try to set the meta data on newfile.mobi.
But the error message is a bit strange. mobi2mobi seem to think that the file have EXTH headers since "if ($exthflg & 0x40)" seems to become true. And that should not happen if it is not a real MobiPocket file. So you could try to add "& false" to this if condition to see if that works.
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