First suggestion would be to put more info in your error message:

rename $tempfile, "tempdir/".$new_filename 
      or die "Cannot rename $tempfile to tempdir/$new_filename" $!";
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You are renaming an open file. This is indeed allowed, but in this code, I see no need and would avoid that.
while ( <$attach> ) makes no sense because $attach is a text string - what you are trying to do is not at all clear.

Update: Also add use strict; use warnings to your code. If you had done that you will see that while ( <$attach> ) produces the error of attempted read on an unopened file handle.