Use the code in Re: Binary search algorithm.. If the returned value is negative, use the closest of $array[~$idx - 1] and $array[~$idx]

my $idx = binsearch { $a <=> $b } $val, @array;
if ($idx < 0) {
   $idx = ~$idx;
   --$idx if $idx > 0 && $val - $array[$idx-1] <= $array[$idx] - $val;
}
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