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...

#!/bin/sh

## 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
fi
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    --k.