I need a simply way to return the hostname associated with an IP.

$ host yahoo.com
yahoo.com has address 66.94.234.13
[ SNIP ]

$ perl -MSocket -le'my $name = gethostbyaddr inet_aton( "66.94.234.13"
+ ), AF_INET; print $name'
w2.rc.vip.scd.yahoo.com
[download]

Regexp::Common

I was looking to have something simple and not have to install any modules.

To clarify: you would not actually have to install Regexp::Common to use one of its regular expressions. You can search for the regular expression that you need, copy the expression out of the module source code, then paste it into your own program.

Erm, just how the frell is a regular expression going to do a reverse lookup on an IP address? I know there were some changes in the engine in 5.10, but really . . .

(That inanity aside, gethostbyaddr that's been pointed out below is the easiest method presuming the underlying OS has a working resolver setup).

The cake is a lie.
The cake is a lie.
The cake is a lie.

Requires:
nmap
webscan.pl
Nmap::Scanner
Net::Nslookup

#!/usr/bin/perl

package WebScanner;

use Net::Nslookup;
use Nmap::Scanner::Util::BannerScanner;
use vars qw(@ISA);

@ISA = qw(Nmap::Scanner::Util::BannerScanner);

my $name = Net::Nslookup->nslookup;
$name = nslookup( domain => 'yahoo.com' );
print $name, "\n";

sub new {

    my $class = shift;
    my $self  = $class->SUPER::new();

    $self->regex('Server:\s*(.+)$');
    $self->send_on_connect("HEAD / HTTP/1.0\r\n\r\n");
    $self->add_scan_port(80);
    $self->add_scan_port(8080);
    $self->add_target( $_[0]  || die "Need target in constructor!\n" )
+;
    
    return bless $self, $class;

}

1;

use lib 'lib';
use strict;

my $web =
  WebScanner->new( $name || die "Missing host to scan!\n$0 host\n" );

$web->register_banner_found_event( sub { shift; print $_[0]->hostname(
+), "\n"; }
);

$web->scan();
[download]

update: OK...to get just the name

#!/usr/bin/perl

use Net::Nslookup;

my $name = Net::Nslookup->nslookup;
$name = nslookup( domain => '66.94.234.13', type => 'PTR' );

print $name, "\n";
[download]