If you only think that, I really think you should check the documentation before you go any further.

Everything that you call, check its return. Everything you do, print a line to see if it worked or died there.

There's stuff you say you "know from debugging" -- why don't you post that stuff here?
--

“Every bit of code is either naturally related to the problem at hand, or else it's an accidental side effect of the fact that you happened to solve the problem using a digital computer.”
M-J D

I also said that " I know that the second Write() call which outputs to STDERR does output a valid pdf file.". I didn't think anybody would find it useful for me to post the contents of the entire pdf file. Here's the first few lines of output produced from the second Write():

%!PS-Adobe-3.0
%%Creator: (ImageMagick)
%%Title: (GLOB(0x92ed52c))
%%CreationDate: (Wed Jan 29 18:07:37 2003)

And here are the last few lines:

%%EndData
%%PageTrailer
%%Trailer
%%BoundingBox: 0 0 611 791
%%EOF


UPDATE: I now realize this isn't what a pdf file looks like. This is what a postscript file looks like. Either I have totally missed the boat on how to use Image::Magick, or it doesn't quite work right for all combinations of conversions yet. In any event, thanks to everybody for their help, I've found a better solution to my problem.