On a sidenote I ran out of memory if I used a simple while loop, which surprised me as that shouldn't slurp the file as say a foreach?

A while loop seems fine to me. It should not slurp all the contents into memory. This prints the 10 millionth line ($.) of a 187M line (2GB) file for me in about 3 seconds:

while (<>) {
    if ($. == 10_000_000) {
        print;
        last;
    }
}
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