If what you want to do is get the name associated to each IP address, you may want to avoid executing nslookup as an external program and use the gethostbyaddr function, which performs the lookup without invoking any external programs. So the line that reads

my $output=`nslookup $array`;

<it>(side note: you were using normal ticks instead of back ticks in your message. This may have been one part of your problem)</it>

would have to become:

my $addr=inet_aton($array);
my $output=gethostbyaddr($addr, AF_INET);
$output=$array unless $output;

the third line sets the output to the IP address if it could not be resolved. You will also have to add the following at the top of your program:

use Socket;

which defines the AF_INET constant.