In case it was not clear from previous posts, your regex was fine, but you matched and then did nothing. You needed to print the match. So your regex:

m/\bDE\d+/g
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would match and stop. What they did (in addition to being more specific with the number of digits) is to group your match in parentheses:

m/\b(DE\d+)/g
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And then the match is located in $1. (Which they print)