The original code from http://www.stevekallestad.com/blog/reverse_ip_lookup_using_perl.html is:

#!/usr/bin/perl
use strict;
se warnings;
use Net::IP;
use Net::DNS;

my $ip = new Net::IP($ARGV[0],4);
print $ip->reverse_ip()."\n";
print "Resolving ...\n";
my $res = Net::DNS::Resolver->new;
my $answer = $res->query($ip->reverse_ip(),'PTR');
my $namer = $answer->{'answer'}[0];
print "PTR Name: $namer->{'ptrdname'}\n";
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@ARGV is a special variable that contains the parameters passed to the script. In the script you copied the ip was passed as a command line option. In your case it is in the variable $a, so you must use this variable in the call to new Net::IP like this:

my $ip = new Net::IP($a,4);
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You better don't change the @ARGV variable at all.

BTW, when you write:

my $ARGV = $a;
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You are assigning the scalar variable $ARGV. And when you write:

my $ip = new Net::IP($ARGV[0],4);
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You are using the array variable @ARGV (its first element)

citromatik