\1 can capture much more than a single character, so, it can't be used inside a character class because it doesn't represent a character, but a string

It's not a problem, see perlretut

   /[\]c]def/; # matches ']def' or 'cdef'
   $x = 'bcr';
   /[$x]at/;   # matches 'bat', 'cat', or 'rat'
   /[\$x]at/;  # matches '$at' or 'xat'
   /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat'
[download]

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" ;
[download]

Output:

ab#c
[download]

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