I'm not sure if this the sort of data structure you are after. It isn't the same as your structure but I think it reflects the hierarchy implied by your data. I have hopefully managed to replicate the sort of data you have in the spreadsheet in my @sanData array.

use strict;
use warnings;

use Data::Dumper;

# @sanData is equivalent to $sheet->Name() and @Column_A data
# in your 'foreach my $sheet' loop.

my @sanData = (
   {
       name => q{SAN_01},
       data => [ qw{
                   RDUB010
                   34:34:36:7U
                   87:23:89:6C
                   RDUB090
                   90:39:12:6U
                   34:34:36:7Z
                   87:23:89:6N
                   34:38:36:7U
                   } ],
   },
   {
       name => q{SAN_02},
       data => [ qw{
                   RDUB030
                   34:7U:34:36
                   87:89:23:6C
                   RDUB070
                   90:12:39:7Z
                   36:34:34:6U
                   89:23:87:6N
                   34:36:38:7U
                   } ],
   },
   );

my $storage = {};

foreach my $rhSheet ( @sanData )
{
    my $name = $rhSheet->{ name };
    $storage->{ $name } = {};
    my $currentLUN = q{};
    foreach my $dataItem ( @{ $rhSheet->{ data } } )
    {
        if ( $dataItem =~ m{:} )
        {
            push
               @{ $storage->{ $name }->{ $currentLUN } },
               $dataItem;
        }
        else
        {
            $currentLUN = $dataItem;
            $storage->{ $name }->{ $currentLUN } = [];
        }
    }
}

print Data::Dumper->Dumpxs( [ $storage ], [ qw{ storage } ] );
[download]

The Data::Dumper output.

$storage = {
             'SAN_01' => {
                           'RDUB090' => [
                                          '90:39:12:6U',
                                          '34:34:36:7Z',
                                          '87:23:89:6N',
                                          '34:38:36:7U'
                                        ],
                           'RDUB010' => [
                                          '34:34:36:7U',
                                          '87:23:89:6C'
                                        ]
                         },
             'SAN_02' => {
                           'RDUB070' => [
                                          '90:12:39:7Z',
                                          '36:34:34:6U',
                                          '89:23:87:6N',
                                          '34:36:38:7U'
                                        ],
                           'RDUB030' => [
                                          '34:7U:34:36',
                                          '87:89:23:6C'
                                        ]
                         }
           };
[download]

I hope this is helpful.