use Socket; 
use strict;
my $addy = shift || die "Enter a IP address";
my $port = getservbyname('smtp', 'tcp');
my $sin = sockaddr_in($port,inet_aton($addy));
my $client = gethostbyaddr($sin, AF_INET); # get name
if ($client)
{
print $client, "\n\a";
}
else
{
print "no\n";
}
[download]

You don't need this sin stuff here. Try your original code, but with the following replacement:

# this:
my $client = gethostbyaddr(inet_aton($addy), AF_INET); # get name

# instead of:
my $client = gethostbyaddr($addy, AF_INET); # get name
[download]

It worked for you with the variable not received from the user because you weren't using quote marks, and you were getting a v-string (a deprecated feature, actually). Your luck was that a v-string is a lot like an inet_aton()-ed string :)

Incidentally, why are you using Socket? Unless you want to use low-level stuff for learning or fine control, you are probably better off using IO::Socket. See perlipc, also.

Thanks!!