I want to know how to extract unique elements in the array

Do you mean that you only want to handle elements which appear only once in the array or that you want to handle all elements (but skip over duplicates)?

A solution for skipping over duplicates could be:

use strict;
use warnings;
use List::MoreUtils qw/uniq/;

my @array = qw/Returns a new list by stripping duplicate values in LIS
+T The order of elements in the returned list is the same as in LIST I
+n scalar context returns the number of unique elements in LIST/;

my @drop_duplicates = uniq grep {length > 4} @array;

{ local $, = "\n"; print @drop_duplicates;}
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And to extract elements that appear only once:

use strict;
use warnings;

my @array = qw/Returns a new list by stripping duplicate values in LIS
+T The order of elements in the returned list is the same as in LIST I
+n scalar context returns the number of unique elements in LIST/;

my %seen;
my @only_once = grep {$seen{$_} == 1 } map {$seen{$_}++; $_} grep {len
+gth > 4} @array;

{ local $, = "\n"; print @only_once;}
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Update: added some code examples.