Topology 
  IPClassA 
  subnet_address
    Topology 
    IPClassA 
    Device = 1
    log_ratio
      Topology 
      IPClassA 
      Device = 1 
      Port = 1 
      poll_interval

  Topology 
  IPClassC 
  security_string
    Topology 
    IPClassC 
    Device = 2 
    Port = 2 
    poll_interval
  

##</code><code>##

use strict;

# Global data structures
use vars qw(@foo @bar);

@bar = qw(Topology IPClassA subnet_address Topology IPClassA Device=1
          log_ratio Topology IPClassA Device=1 Port=1 poll_interval Topology 
          IPClassC security_string Topology IPClassC Device=2 Port=2 
          poll_interval);

my $i = 0;
my $j = 1;

print "Foo Before:\n\n";
foreach $_ (@bar){
    print "$_\n";}

push @foo, "Topology";
    foreach $_ (@bar)
    {
        if ($bar[$i] eq $bar[$j])
        {
            $i ++;
	    $j ++;
	}
	else
	{
	    push @foo, $bar[$j];
	    $i  = 0;
	    $j ++;
	}
    }
	    print "\nFoo After:\n\n";
	    foreach $_ (@foo){
	        print "$_\n";}
  

##</code><code>##

Foo Before:

Topology
IPClassA
subnet_address
Topology
IPClassA
Device=1
log_ratio
Topology
IPClassA
Device=1
Port=1
poll_interval
Topology
IPClassC
security_string
Topology
IPClassC
Device=2
Port=2
poll_interval

Foo After:

Topology
IPClassA
subnet_address
Device=1
log_ratio
Device=1
Port=1
poll_interval
IPClassC
security_string
IPClassC
Device=2
Port=2
poll_interval

  

##</code><code>##

Foo After:

Topology
IPClassA
subnet_address
Device=1
log_ratio
Port=1
poll_interval
Topology
IPClassC
security_string
Device=2
Port=2
poll_interval