use Scalar::Util qw(looks_like_number);

sub hostname_by_num { 
    my $num = shift;

    # validate our argument
    defined $num
       or return undef;
    $num >= 1
       or return undef;
    looks_like_number( $num )
       or return undef;

    $num--;  # convert into array index.

    return exists $HOSTS[$num] ? $HOSTS[$num] : undef;
}