Thanks for the compliments on my help message text. I do try to be clear.

You may have a point, re: your replacement code, with the exception that eliminating the subroutine definition for HELP_MESSAGE() would cause the script to fail to produce the contents of $help when invoked with the --help option. Thus, my two explicit conditional invocations of HELP_MESSAGE() should probably be replaced as you indicated, though I'd keep the subroutine. New code:

die $help unless $ARGV[1];
map { /^-?\d+$/ or die "Invalid input $_\n" } @ARGV;

# . . . and later . . .
sub HELP_MESSAGE {
  print $help;
}
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If I don't keep that subroutine in there, the --help option causes the following output to be produced:

/home/ren/bin/ies version 1.0 calling Getopt::Std::getopts (version 1.
+05),
running under Perl version 5.8.8.


Usage: series [-OPTIONS [-MORE_OPTIONS]] [--] [PROGRAM_ARG1 ...]


The following single-character options are accepted:
        Boolean (without arguments): -s

Options may be merged together.  -- stops processing of options.
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edit: johngg pointed out the "unless" bug above, where unless @ARGV should read unless defined @ARGV instead.