#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11161117
use warnings;

for ( qw( a aa bbb cccc caab deefffgggghhiiiii aabbbccdee wwxxyyzz ) )
  {
  printf "%20s -->", $_;
  print " $2" while /(?=(?:^|(.)(?!\1))(([a-z])\3)(?!\3))/gi;
  print "\n";
  }
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Outputs:

                   a -->
                  aa --> aa
                 bbb -->
                cccc -->
                caab --> aa
   deefffgggghhiiiii --> ee hh
          aabbbccdee --> aa cc ee
            wwxxyyzz --> ww xx yy zz
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