In my experience, at least on some platforms, truncate right after print may fail. You need an intermediate seek to be safe.

    print SELF new_crc($file),$/;
    seek SELF, 0, 1;
    truncate(SELF, tell SELF);
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See also perldoc -f seek:

Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing.

Perhaps you don't need the truncate — or the seek. CRCs tend to all be the same string length.