die("Usage: $0 [-c] [-v] <-f pattern-file | subnet/netmask> file [file...]\n")  unless ( (defined $#ARGV && $#ARGV > -1) || (defined $opt_f) ) ;
...
if ( defined $#ARGV && $#ARGV > -1) {

##</code><code>##

die "Usage: $0 [-c] [-v] <-f pattern-file | subnet/netmask> file [file...]\n" unless @ARGV || defined $opt_f;
...
if ( @ARGV ) {

##</code><code>##

			# remove non-ip-address characters, trim, break into words to validate
			$IP_LINE =~ s/[^0-9.]+/ /g ;
			$IP_LINE =~ s/^ // ;
			$IP_LINE =~ s/ $// ;
			my @WORDS = split(/\s+/,$IP_LINE) ;

##</code><code>##

			# remove non-ip-address characters, trim, break into words to validate
			$IP_LINE =~ tr/0-9./ /c;
			my @WORDS = split ' ', $IP_LINE;

##</code><code>##

	# grep each file
	for my $FNAME (@ARGV) {
		if ( ! -r $FNAME ) {
			printf STDERR "Cannot read: $FNAME\n" ;
			next ;
		}

		open ($INFILE, "<$FNAME") ;

##</code><code>##

	# grep each file
	for my $FNAME (@ARGV) {
		open my $INFILE, '<', $FNAME or do {
			warn "Cannot open '$FNAME' because: $!";
			next;
		};