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+#!perl
+use strict;
+use warnings FATAL => qw(all);
+
+=prompt
+You are given a string, $str, consisting of lowercase English letters.
+
+Write a script to return the final string after all duplicate removals have been made. Repeat duplicate removals on the given string until we no longer can.
+
+    A duplicate removal consists of choosing two adjacent and equal letters and removing them.
+
+
+Example 1
+
+Input: $str = 'abbaca'
+Output: 'ca'
+
+Step 1: Remove 'bb' => 'aaca'
+Step 2: Remove 'aa' => 'ca'
+
+
+Example 2
+
+Input: $str = 'azxxzy'
+Output: 'ay'
+
+Step 1: Remove 'xx' => 'azzy'
+Step 2: Remove 'zz' => 'ay'
+
+
+Example 3
+
+Input: $str = 'aaaaaaaa'
+Output: ''
+
+Step 1: Remove 'aa' => 'aaaaaa'
+Step 2: Remove 'aa' => 'aaaa'
+Step 3: Remove 'aa' => 'aa'
+Step 4: Remove 'aa' => ''
+
+
+Example 4
+
+Input: $str = 'aabccba'
+Output: 'a'
+
+Step 1: Remove 'aa' => 'bccba'
+Step 2: Remove 'cc' => 'bba'
+Step 3: Remove 'bb' => 'a'
+
+
+Example 5
+
+Input: $str = 'abcddcba'
+Output: ''
+
+Step 1: Remove 'dd' => 'abccba'
+Step 2: Remove 'cc' => 'abba'
+Step 3: Remove 'bb' => 'aa'
+Step 4: Remove 'aa' => ''
+=cut
+
+use Test2::V0 -no_srand => 1;
+is(duplicate_removal('abbaca'), 'ca', 'Example 1');
+is(duplicate_removal('azxxzy'), 'ay', 'Example 2');
+is(duplicate_removal('aaaaaaaa'), '', 'Example 3');
+is(duplicate_removal('aabccba'), 'a', 'Example 4');
+is(duplicate_removal('abcddcba'), '', 'Example 5');
+done_testing();
+
+sub duplicate_removal
+{
+     my $str = $_[0];
+     while ( $str =~ /(.)\g{1}/ )
+     {
+          $str =~ s/(.)\g{1}//;
+     }
+     return $str;
+}