Foo Before:

  Topology
  IPClassA
  subnet_address=0
  Topology
  IPClassA
  Device
  model_name=1
  Topology
  IPClassB
  Device
  model_name=2
  
##</code><code>##

use strict;

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

@bar = qw(Topology IPClassA subnet_address=0 Topology IPClassA Device 
          model_name=1 Topology IPClassB Device model_name=2);

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 After:

Topology
IPClassA
subnet_address=0
Device
model_name=1
IPClassB
Device
model_name=2