You also want to make up your mind on whether @lines is being modified in place. If so, then you don't need @newlines at all and it's just

s/#(?=<-\(0N<-\(s3T)/*/g foreach ( @lines );
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Otherwise, if you need to keep both versions around, then it's

@newlines = map { s/#(?=<-\(0N<-\(s3T)/*/gr } @lines;
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(it being somewhat more efficient to allocate @newlines in one swell foop).