Maxim Mironjuk
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October 14, 2025
Many companies still run business critical core systems in COBOL, Perl, or other languages whose last complete documentation dates back decades, with the original authors long gone from the company. A full ground up rewrite sounds tempting on paper, but in practice it regularly fails because of underestimated business logic that grew over years. This article shows how to use Claude deliberately to first understand such systems, extract their rules, and then replace them in a controlled way, module by module, instead of in one single risky step.