I've never used it, but Net::Nslookup seems to do what you want.

my $name = nslookup(host => $ip, type => "PTR") || $ip;
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I'd like to make three suggestions:

1) Cache the results of your queries so you do lookup the same IP addresses over and over again. This will greatly improve your script's performance.

if (!$name_cache{$ip}) {
   $name_cache{$ip} = nslookup(host => $ip, type => "PTR") || $ip;
}

my $name = $name_cache{$ip};
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2) You may want to insert a slight pause between lookups to give the DNS server a chance to breath. I don't know the proper etiquette in this situation.

3) You may want to make the reverse lookup optional. It can put a fair load on the DNS server, and it can slow down your program greatly.