when you say "if one exists" does that mean that from the url 'http://peeron.com/inv' you don't want to get anything? Or do you want to get 'peeron.com'? or maybe '66.216.39.12'?

for the simplest case, you just want to get the ip from a url like 'http://66.216.39.12/inv'. A simple regex to do this would be:

$url = "http://66.216.39.12/inv";
$url =~ m!https?://((?:\d{1,3}\.){3}\d{1,3})!i;
$ip = $1;
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of course, this will also match things like "http://999.999.999.999/", but I'm not going to worry about that.

For the more complex case, where you actually want the real ip, given the hostname, I'd use Net::DNS. Shamelessly copied from the Net::DNS manpage

use Net::DNS;

my $url = "http://peeron.com/inv";
$url =~ m!https?://([^/]+)!i;

my $res   = Net::DNS::Resolver->new;
my $query = $res->search($1);
  
if ($query) {
    foreach my $rr ($query->answer) {
        next unless $rr->type eq "A";
        print $rr->address, "\n";
    }
} else {
    print "query failed: ", $res->errorstring, "\n";
}
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Again, the usual warnings - this assumes you have a valid URI, and not something that will buffer overflow your DNS server, for example.

update: or look below at rob_au's reply, for the really clean solution.

-- Dan