Ok - here is a functional decomposition of "my @uniqueIPs = grep !$counts{$_}++, @ipAddresses;":

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

#  The resulting value in %count is equivalent  to the  execution of t
+his 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 c
+reated, 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-increm
+ent($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,  an
+d  post-negation  will be zero.
#  "grep" filters out,  leaving only non-zero values, effectively retu
+rning all  the KEYS of %counts, which  is
#  all the unique IP's.
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