in reply to Detecting when my ISP changes my IP
You don't say why you need to detect an IP change, but there's been a few similar posts here in the monastery that might be worth checking out.
Another option if you use ISC's dhclient, is to forego Perl altogether and make use of dhclient-script's exit hooks with an (untested!) shell script like...
## Script is sourced from dhclient-script.
## $reason and $new_ip_address are set there.
if [ "X$reason" = "XBOUND" ]; then
echo $new_ip_address > ~adamm/.current_inet_addr