my @uniqueIPs; #Just declare it as an empty array;

#  The resulting value in %count is equivalent  to the  execution of this statement:
$counts{$_}++  for @ipAddresses;  # This is  the important piece.

# The hash %counts uses each ipAddress  as a key.
# The first time an IP address is encountered, the key-value pair is created, with a value of zero.
# the(++) increments that to 1.
# The next time  that IP is encountered, the value is incremented. So, at the end, the value for each  key
# contains the count of occurrances for that IP address (key).

... grep !$counts{$_}++, @ipAddresses
# The "grep:searches  through  each value ($counts{$_} is the VALUE), and the negation  (!)
# looks for non-zero values.  Because the operator used is post-increment($blah++), before negation,
#  the value returnedwill be zero  for First-seen  IP addresses only. For second and subsequent
#  sightings of the IP,  the pre-negation value will  be non-zero,  and  post-negation  will be zero.
#  "grep" filters out,  leaving only non-zero values, effectively returning all  the KEYS of %counts, which  is
#  all the unique IP's.