What did you expect $c++ to do, knowing that the first time the code is run the value of $c is the current regular expression ?

Anyway, here is something that should do what you want: 

'234565432' =~ / 
  (\d) (?{ $c = $1}) # Match the first number and initialize $c
  (?:
    (??{ ++$c })     # Same as the previous char + 1
  )*/x;

print $&;

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In reply to Re: Regex to match ascending sequence by Eily
in thread Regex to match ascending sequence by ExReg

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