Please read How do I post a question effectively?. In particular, please wrap code, input and output in <code> tags.

There are multiple ways to achieve your desired result, but I would do it using Look Around Assertions. In your case, your code might look like:

$segment =~ s/^\d{2}(?!\d)/testtext/;
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which YAPE::Regex::Explain describes as

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
----------------------------------------------------------------------
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Some side notes:

• The language is Perl, the interpreter is perl; there is no 'PERL' (excluding when I get very excited about Perl. And who doesn't?)
• If this is class-work, please qualify it as such. We are all happy to help beginners learn and help you debug, but Monks doing work for you won't help you learn. It also tends to raise ire in the Monastery.