Tried using strict? It's a good habit to aquire.
I think what you're trying to do can be done with this:

use strict;
open(NB, "nbt.csv") or die "Error reading nbt.csv: $!\n";

while(<NB>){
   chomp;
   @nbtstat = `nbtstat -a $_`;
   %_=map{$_,1}map{/^\d+.\d+\.\d+\.\d+/;$&}($_,@nbtstat);
   print join(',',$_,keys(%_)),"\n";
}
[download]

As I understand it, you want:

1. nbtstat each IP address in the file (hopefully XXX.XXX.XXX.XXX, 15 chars ;)
2. Make a list of unique IP addresses (found at the beginning of each line returned)
3. Print the list to STDOUT comma seperated

Or have I got the wrong end of the stick?

Note: this'll print the original IP address twice.

--

Brother Frankus.

¤