I'm not sure this is really "help," but rewriting like the following may help you track it down:

#!c:\perl\bin\perl -w

use strict;  # you know you wanted this, right?

open(NB, "nbt.csv") or die "Can't open input nbt.csv\n";
# NBT.CSV simply contains ip addresses - one per line

while(<NB>)
    {
    chomp $_;
    my $line="$_".",";      # using my eliminates the undef
    my @nbtstat = `nbtstat -a $_`; # my will make you happy

    for ($j=0; $j<=$#nbstat;$j++)  # $# is the last index
        {
        my $short=substr($nbtstat[$j],0,15);
        chomp $short;
        unless ($line=~m/$short/)
            {$line=$line.",".$short;}
        }
    print "$line\n";
    }
close(NB);
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Update: Could we see an example of the output you are trying to achieve (generated by hand if necessary)?