$segment =~ s/^\d{2}(?!\d)/testtext/;

##</code><code>##

NODE                     EXPLANATION
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  \d{2}                    digits (0-9) (2 times)
----------------------------------------------------------------------
  (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
    \d                       digits (0-9)
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------