m/^        # Match the start of the line
    (\w)     # Match the first letter, capture it in $1 (or \1 if you re-use it in the regex)

    (        # Open capturing group for $2
      (?!\1)   # Look ahead and verify that the next charachter isn't the same as the first one
      \w       # Capture a word-charachter (because of the look ahead this can't be the same one as the first)
    )        # Close capture $2

    \2       # Match the second charachter another time (no need to capture this)

    (        # Open capture $3
      (?!       # Open look-ahead
         \1       # Charachter 1
        |         # Or (don't use a charachter class, else you run in the same trouble as before)
         \2       # Charachter 2
      )         # Close look-ahed
      \w      # Match a word-charachter
    )       # Close capture $3
...
   /x;