If you don't insist on using XML::LibXML you could do something like this:

use strict;
use XML::Rules;

my $parser = XML::Rules->new(
    stripspaces => 7,
    rules => [
        '_default' => 'content',
        'Dev_Info' => sub {
            print "$_[1]->{dev_name}\t$_[1]->{configuration}\n";
            return;
        },
        'Device' => '',
    ],
    start_rules => [
        'Hyper,RAID-5_Device,Back_End,Mirror_Set,Front_End,Product,Lab
+el,Flags,Capacity' => 'skip',
    ],
);

$parser->parse(\*DATA);

__DATA__
<?xml version="1.0" standalone="yes" ?>
<SymCLI_ML>
...
[download]

or (to match the second example)

use strict;
use XML::Rules;

my %conf_of;
my $parser = XML::Rules->new(
    stripspaces => 7,
    rules => [
        '_default' => 'content',
        'Device' => sub {
            $conf_of{$_[1]->{Dev_Info}{dev_name}} = "$_[1]->{Dev_Info}
+{configuration},$_[1]->{Capacity}{cylinders}";
            return;
        },
        'Dev_Info,Capacity' => 'no content',
    ],
    start_rules => [
        'Hyper,RAID-5_Device,Back_End,Mirror_Set,Front_End,Product,Lab
+el,Flags' => 'skip',
    ],
);

$parser->parse(\*DATA);
use Data::Dumper;
print Dumper(\%conf_of);

__DATA__
<?xml version="1.0" standalone="yes" ?>
<SymCLI_ML>
...
[download]

or to get rid of the global variable something like

use strict;
use XML::Rules;

my %conf_of;
my $parser = XML::Rules->new(
    stripspaces => 7,
    rules => [
        '_default' => 'content',
        'Device' => sub {
            return $_[1]->{Dev_Info}{dev_name} => "$_[1]->{Dev_Info}{c
+onfiguration},$_[1]->{Capacity}{cylinders}";
        },
        'Dev_Info,Capacity' => 'no content',
        'Symm_Info' => sub {return symid => $_[1]->{symid}},
        'Symmetrix' => 'no content',
        'SymCLI_ML' => 'pass',
    ],
    start_rules => [
        'Hyper,RAID-5_Device,Back_End,Mirror_Set,Front_End,Product,Lab
+el,Flags' => 'skip',
    ],
);

my $conf = $parser->parse(\*DATA);
use Data::Dumper;
print Dumper($conf);

__DATA__
<?xml version="1.0" standalone="yes" ?>
<SymCLI_ML>
...
[download]

Your requirements do look like something that XML::Rules was designed for.