shadrack gave a good answer by using gethostbyaddr with the pack function to no use the Socket module which is tricky.
We normally would do :

 
#!/usr/bin/perl
use Socket;

#Resolve IP address into hostname
sub ReverseLookup {
    my(@addrs,$xname,$xaliases,$xtype,$xlen,$addr);

#    @addrs=split(/\./,$_[0]);
#    $addr=pack('c4',@addrs[0..3]); # c or C - A signed/unsigned char 
+(8-bit integer) value
     $addr = inet_aton($_[0..3]);
    ($xname,$xaliases,$xtype,$xlen,@addrs)=gethostbyaddr($addr,2);
    return $xname;
}
[download]

We don't know why MonkeyManChfKiller don't want to use modules, it's his choice but obviously this can be achieved in another way with perl.
Nice use of pack() which I don't see very often.