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";
}
...