Given what Michael says about the log entries, it looks as if $Sender->Attach() is the location of interest.

There are some classes of files that can block on reading, such a FIFO on Unix. Try a quick ls (or dir on windows) to peek at the file. Some variant of the following scrap

-e $attachfile and do{
   $ENV{f}=$attachfile; 
   system('(echo file is "$f"; ls -l "$f" ) 2>&1 >> EXTRA_DEBUG_LS_OUT
+PUT')
};
[download]

But it's funny: I wouldn't use fifo's in mailing/pdf-creation scenarios.

My other guess would be that we've e.g. '|' at the end of name and the module allows unsafe 2-argument open. In this case, the forked process creating the pdf might hang (check for child processes, e.g. with ps).

But that depends on some still missing parts of the big picture surrounding what you've already told us.

HTH
Peter

PS: Micheal, <READMORE> is a good friend of us monks and helps with browsing and saving screen real-estate by allowing optional display of long listings :). Thx!