Actually, I think I'd prefer not using Getopt::Long (or even Getopt::Std) at all: just expect one argument after the name of the script. If it turns out to be the name of an existing file, read that file; if it's not a file, but it looks like an IP range, treat it as such; otherwise die with a usage message:

use strict;

my $Usage = "$0 lower.ip.addr-upper.ip.addr\n  or\n$0 text.file\n";

die $Usage unless @ARGV == 1;

my ($iplower, $ipupper);
if ( -f $ARGV[0] ) {
   # treat the arg as a file
}
elsif ( $ARGV[0] =~ /^((?:\d{1,3}\.){3}\d{1,3})-(\d{1,3}(?:\.\d{1,3}){
+3})$/ ) {
   ($iplower,$ipupper) = ($1,$2);
}
else {
   die "Invalid command-line arg: $ARGV[0]\n\n$Usage";
}
...
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(update: I do not mean to suggest that I'd never use of the Getopt modules -- I do use them often. It's just that for a simple case like this, option flags seem unnecessary: you have to provide one type of arg or the other, and you can't provide both -- these are not really "options", and you don't need a command-line flag to tell them apart.)