Reading this reminded me .. I had the exact same thing trip me up. -f is not a switch to the grep function but rather an independently defined function that just happens to be used as an argument here. But the unix-familiar mind sees that and thinks "grep -f" is a special form of "grep" and seeks the answer in the grep documentation. And doesn't find it.

Using the block form of grep might have made this clearer.

@files = grep { -f } glob("~/data/*");
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