It is a good idea to check for the success of your opens and closes and to use the three argument form for open with lexical filehandles. You should also use single quotes if you don't need interpolation.

my $hostfile = '/tmp/jn/hosts';
open my $hostFH, '<', $hostfile
   or die "open: < $hostfile: $!\n";
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You need to take care when sorting IPs as a simple lexical sort will give odd results.

$ perl -le '
> @IPs = qw{ 22.32.87.12 56.67.38.61 101.12.34.54 };
> print for sort @IPs;'
101.12.34.54
22.32.87.12
56.67.38.61
$
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I use sprintf here to form each quad into a fixed width string with leading zeros, making a 12-digit string that can be sorted lexically.

use strict;
use warnings;

my %seen;
my @sortedUniques =
   map  { ( split m{\s+}, $_->[ 0 ] )[ 0 ] }
   sort { $a->[ 1 ] cmp $b->[ 1 ] }
   grep { ! $seen{ $_->[ 1 ] } ++ }
   map  {
            m{^(\d+)\.(\d+)\.(\d+)\.(\d+)\t}
            ? [ $_, sprintf q{%03d%03d%03d%03d}, $1, $2, $3, $4 ]
            : ()
        }
   <DATA>;

print qq{$_\n} for @sortedUniques;

__END__
# hosts file

10.31.17.65    fw2
192.168.1.78    hosta
192.168.1.21    hostb
10.31.17.65    fw2dup
10.23.212.6    hostc
192.168.100.254    fw1
192.168.1.21    hostbdup
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The output.

10.23.212.6
10.31.17.65
192.168.1.21
192.168.1.78
192.168.100.254
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I hope this is helpful.