@test =(1,2,2.5,3.5,4.5,7,6,-5,-6,-7,7,8,9,10,0,12,18);


sub range {
   @_=sort{$a<=>$b}@_;@r=($l)=$_[0];
   map{if(abs$l-$_>1){push@r,$r=$_;
   $r[-2].="~$l";}$l=$_;}@_;@r;
}

@result = range(@test);
print "$_\n" for @result;


Prints:
-7~-5
0~4.5
6~10
12~12
18