Hmmm. The strange thing is I actually had a 'rewind' in there at some point but removed it again because it didn't have any impact. However if I change the first copy operation to

  copy $src_file, $tmp1_fh
    or die "$ERRNO\n";
  seek $tmp1_fh,0,0;
[download]

then it magically works now. Thanks for the tip! Maybe there is some other bug lurking in the original program, but now that the basic copy works I'm sure to figure it out.