Avoiding the assumption that hostnames start with "H", and simplifying logic:

#! perl -slw
use strict;
use Data::Dumper;

my( @data, $current );
while( <DATA> ) {
    if( my ($host,$ip,$brand) =/^(\w+)\s+(\S+)\s+(\S+)/ ) {
        $current and push @data, $current;
        $current = {HOST=>$host, IP=>$ip, BRAND=>$brand};
        next;
    }
    my( $key, $val )  = m[\s+(.+)\s+(\S+)$] or next;
    $current->{$key} = $val;
}
$current and push @data, $current;

print Dumper \@data;

__DATA__
Hostname1     1.1.1.1     Cisco
              Chassis Serial Number xyz123
              Interface Gig0/0/31
Hostname2     2.2.2.2     Juniper
              Chassis Serial Number abc123
              Interface Gi-0/0/31
[download]

Output:

$VAR1 = [
          {
            'Chassis Serial Number' => 'xyz123',
            'Interface' => 'Gig0/0/31',
            'BRAND' => 'Cisco',
            'HOST' => 'Hostname1',
            'IP' => '1.1.1.1'
          },
          {
            'HOST' => 'Hostname2',
            'BRAND' => 'Juniper',
            'Chassis Serial Number' => 'abc123',
            'Interface' => 'Gi-0/0/31',
            'IP' => '2.2.2.2'
          }
        ];
[download]

Alternative, using hostname as a key to a hash:

 my ($current,%data)
...
        $current and $data{$current->{HOST}} = $current; # in 2 places
        $current = {HOST=>$host, IP=>$ip, BRAND=>$brand};
[download]