Just redirect stderr into a file. See also the log_file method.

       $object->log_file("filename" | $filehandle | \&coderef | undef)
           Log session to a file.  All characters send to or received 
+from the
           spawned process are written to the file.  Normally appends 
+to the
           logfile, but you can pass an additional mode of "w" to trun
+cate the
           file upon open():
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