Nope he is correct, it was a problem with 'and' having lower precedence than ','

grep ( ( expression) and ( expression) and ( expression ) , @files )
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Is interpreted as

grep ( ( expression) and ( expression) and ( ( expression ) , @files )
+ )
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According to perldoc -f grep, the construct grep( expression, list ) is allowed. But it is indeed confusing to man and machine!.