#!usr/bin/perl
use warnings;
use strict;
use Data::Dumper;

use List::Pairwise qw(mapp grepp); ##need to install this module

my @ary = qw(AirDefense 2 Mobile-N-Site 4 Owl 6 undef 8);
my %hash = mapp{$a ne 'undef' ? ($b,$a):()}@ary;

foreach my $opt (sort{$a<=>$b} keys %hash)
{
    print "$opt => $hash{$opt}\n";
}
__END__
2 => AirDefense
4 => Mobile-N-Site
6 => Owl