try this

lanx@nc10-ubuntu:~$ perl -de0
...
  DB<1> $a="aaAabBAAa";
 
  DB<2>  print join " ",($a =~ /((.)\G\2|(.)\3)/gi) 
aa  a aA a  Aa A  bB  b AA  A Aa A
[download]

there should be a way to backreference matches without listing them afterwards, but I have no time to look it up.

otherwise:

  DB<3>  print $&," " while ($a =~ /(.)\G\1|(.)\2/gi) 
aa aA Aa bB AA Aa
[download]

Hope it helps!