in reply to Re^2: \1, \2, \3, ... inside of a character class
in thread \1, \2, \3, ... inside of a character class

Ok, what you want is \1 working as a group of chars. Well, we can't, but you can use some complexer code: 
  $var = "abXaaabbbc" ;
  $var =~ s/(..)X(??{"[$1]+"})/${1}#/ ;
  print "$var\n" ;

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Output: 
ab#c

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Graciliano M. P.
"Creativity is the expression of the liberty".

In Section Seekers of Perl Wisdom