This is not a good solution. This is quite a poor solution. You shouldn't be checking if the value is an array reference. It should always be an array reference.

use List::Util    qw( uniq );
use Sort::Natural qw( natsort );

my %data;
while ( <$fh> ) {
   chomp;
   my ( $ip, @hosts ) = split;
   push @{ $data{ $ip } }, @hosts;
}

for my $ip ( natsort keys( %data ) ) {
   my @hosts = natsort uniq @{ $data{ $ip } };
   say join "\t", $ip, @hosts;
}
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Actually, I'd use a hash of hashes since it will handle duplicates. The code would look something like this:

use Sort::Natural qw( natsort );

my %data;
while ( <$fh> ) {
   chomp;
   my ( $ip, @hosts ) = split;
   ++$data{ $ip }{ $_ } for @hosts;
}

for my $ip ( natsort keys( %data ) ) {
   my @hosts = natsort keys %{ $data{ $ip } };
   say join "\t", $ip, @hosts;
}
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It needs to handle blank lines and comments, but you get the idea.

You don't need to use natsort, but it provides a better sort order than sort. In fact, you don't have to use any sorting at all, but it's nicer.