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) {
[download]

The variable $#ARGV is ALWAYS defined!    You probably want something like this instead:

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

            # 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) ;
[download]

Would be better as:

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

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

        open ($INFILE, "<$FNAME") ;
[download]

You have the possibility of a race condition where the file could become non-readable after the -r test.    You should print the error message only if open fails and include the $! variable so you know why it fails.

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

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