I disagree with tilly that using sort() is the natural solution. I think this is the natural solution:

sub closest {
$n=pop;$c=pop;abs$n-$_<abs$n-$c?$c=$_:0for@_;$c
}
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This is O(n) rather than O(n*log n). Of course, at 47 characters it is, unfortunately, a longer solution. Oh well!