Regex can be used for validation or data extraction. You want to perform the latter, so no need to examine the data that closely. You have tab-separated data, so all you need to match your data is:

        _____________________ date
       /       ______________ desc
      /       /       _______ amount
     /       /       /
  ------  ------  ------
/^[^\t]*\t[^\t]*\t[^\n*]\n\z/
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You want to remove the amount, so

s/^[^\t]*\t[^\t]*\t\K[^\n*]//;     # 5.10+

s/^([^\t]*\t[^\t]*\t)[^\n*]/$1/;   # Any version
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