First of all, after re-formatting, adding the usual use strict; use warnings; and adding the missing use Net::DNS::Resolver and adding the missing variables, i get Net::DNS::Resolver->new(): nameservers must be an arrayref.

Let's fix that and add some actual nameservers and an IP to query:

#!/usr/bin/env perl

use strict;
use warnings;

use Net::DNS::Resolver;

my $l_ip = '216.92.34.251'; # one of the perlmonks.org servers
my @nameservers = qw[8.8.8.8 8.8.4.4]; # Googles DNS Servers

my $l_dns_name = $l_ip;

my $l_dns = Net::DNS::Resolver->new( nameservers => \@nameservers );
my $l_response = $l_dns->query($l_ip);

if($l_dns->errorstring eq 'NOERROR') {
    my $l_record;

    foreach $l_record ($l_response->answer()) {
        print 'Got: ', $l_record->type(), '  ->  ', $l_record->name(),
+ "\n";
        if($l_record->type() eq 'PTR') {
            $l_dns_name = $l_record->name();
            last;
        }
    }
} else {
    print $l_dns->errorstring, "\n";
}
[download]

This gives us the result Got: PTR  ->  251.34.92.216.in-addr.arpa

Took me a while playing with the code and reading the docs to find out the problem. Turns out that you have to use type-specific methods to parse the part of the answer you actually want. The complete answer from the nameserver is something like this record:

251.34.92.216.in-addr.arpa. 6883 IN    PTR    perlmonks.org.
[download]

name() returns the first part, the name ("key") of that record. But what you want is the second part (the "value"), for which you need type-specific query functions, in this case ptrdname() from the Net::DNS::RR::PTR module.

So, here's what i finally ended up with:

#!/usr/bin/env perl

use strict;
use warnings;

use Net::DNS::Resolver;

my $destip = '216.92.34.251'; # one of the perlmonks.org servers
my @nameservers = qw[8.8.8.8 8.8.4.4]; # Googles DNS Servers

my $ptr = getHostname($destip, \@nameservers, 'PTR');
print "Got name $ptr\n";

sub getHostname {
    my ($l_ip, $nameservers, $recordtype) = @_;
    
    my $l_dns_name = $l_ip;
    
    my $l_dns = Net::DNS::Resolver->new( nameservers => \@nameservers,
+ $recordtype );
    my $l_response = $l_dns->query($l_ip);
    
    if(!defined($l_response)) {
        print "Error: Got no response\n";
        return $l_ip;
    }
    
    if($l_dns->errorstring eq 'NOERROR') {
        my $l_record;
    
        foreach $l_record ($l_response->answer()) {
            print 'Got: ', $l_record->type(), '  ->  ', $l_record->nam
+e(), "\n";
            if($l_record->type() eq 'PTR') {
                $l_dns_name = $l_record->ptrdname();
                #$l_dns_name = $l_record->address();
                last;
            }
        }
    } else {
        print $l_dns->errorstring, "\n";
    }
    
    return $l_dns_name;
}
[download]

Got: PTR  ->  251.34.92.216.in-addr.arpa
Got name perlmonks.org
[download]

Be aware: Not every IP will resolve to a hostname. You will need to add appropriate error handling as well as handling Nameserver connection and lookup errors of all kinds.