From another forum I have learned that perldb requires STDOUT to be a terminal, so that the ">../to_excel/excnnnnn.txt" in my invocation line caused my problem. This would not have to be the case: the writer on the site where I learned about this feature noted that perldb could find the terminal from any of STDIN, STDOUT, or STDERR, whichever was not redirected.
As it is, I will revise the program to open and use the output file named in ARGV[0], and leave my command line unchanged except for replacing '>' with a space.