The specification is simple: given a list of numbers, return the two which are nearest to one-another. Assume that the list contains at least two numbers, and the return order is irrelevent. Some foolhardy attempts of my own follow: 
# 114 chars
sub nn1 {
  my@p;my$d;for(0..$#_-1){for my$n ($_+1..$#_){my$e=abs$_[$_]-$_[$n];i
+f(!defined$d or$e<$d){$d=$e;@p=@_[$_,$n];}}}@p
}

# 100 chars
sub nn2 {
  my@l=sort@_;$l[$_]=[@l[$_,$_+1]]for 0..$#l;pop@l;@{(sort{abs$$a[0]-$
+$a[1]<=>abs$$b[0]-$$b[1]}@l)[0]}
}
  
# 87 chars
sub nn3 {
  @_=sort@_;my%d;$d{abs$_[$_]-$_[$_+1]}=[@_[$_,$_+1]]for 0..$#_-1;@{$d
+{(sort keys%d)[0]}}
}

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Update: regarding the issue of duplicates, assume they are pairs like any other, so you would return duplicate numbers if they were present in the list. 
   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

In reply to (Golf) Nearest Neighbors by MeowChow

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