my %looked_up;
my @urls = qw( list of URLS however you got them );
my @ips;

foreach ( @urls ) {
    my $hostname = extract_hostname_from_url( $_ );
    unless ( exists $looked_up{ $hostname ) {
        my $packed_ip = gethostbyname( $hostname );
        if (defined $packed_ip) {
            $ip_address = inet_ntoa($packed_ip);
            push @ips, $ip_address;
            $looked_up{ $hostname } = 1;
        }
    }
}

# do with @ips whatever you were going to do with them