use Net::Nslookup;

open(INFILE, 'os.txt') || die("cannot open file: ".$!);
my @lines = <INFILE>;

my $switch;

foreach $switch(@lines) {

  my @output = nslookup(host => $switch, 
                        server => 10.1.10.42,
                        type => "A");
  (my $name, my $ip) = (split /:/, $output[3]);
  print "$ip\n";
}
close INFILE;