sub fract {
        %_=(%_,$_[0],$%);
        ~~%_
    }
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This is bogus. $% is a built-in variable meaning "the current page number of the output." You could also write it as $_{$_[0]} = 1; which has a similar effect on the contents of the %_ hash.

The ~~%_ is the same as scalar(%_) because the ~ is the bitwise not. A hash in scalar context becomes a string representing the fraction of used hash buckets over the unused buckets.