IMHO the best data structure is an array of hashes. Each time you encounter a hostname line, you push a new hash onto the array and store the hostname. For each of the following lines you can refer to that hash by $array[-1] until the next hostname line. See below:

use strict;
use warnings;

my @results;
while(<DATA>){
  push @results, {'hostname' => $1}         if /^(hostname.*)/; # push
+ a new hash into the array
  $results[-1]->{'vlan'}+= split(/\s+/) - 3 if /^vlan/;         # numb
+er of words - 3
  $results[-1]->{'ifconfig'}++              if /^ifconfig/;     # one 
+for each ifconfig
}

for( @results ) {
  print "Hostname :  $_->{'hostname'}\n";
  print "# of vlans: $_->{'vlan'}\n";
  print "# of ifconfig lines: $_->{'ifconfig'}\n\n";
}

__DATA__
# hostname 123
hostname 123
vlan create vif1 1 2 3 4 5 6
vlan add vif1 7 8 
ifconfig vif1-1 some data here
ifconfig vif1-2 some data here
ifconfig vif1-9 some data here
# hostname 456
hostname 456
vlan create vif5 1 2
vlan add vif5 3 4 5
vlan add vif5 6 7 8 9
ifconfig vif5-1 some data here
ifconfig vif5-2 some data here
ifconfig vif5-9 some data here
[download]

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