# 114 chars
sub nn1 {
  my@p;my$d;for(0..$#_-1){for my$n ($_+1..$#_){my$e=abs$_[$_]-$_[$n];if(!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]}}
}