#!/user/bin/perl

use strict;
# use warnings;

# SETUP THE FILES FOR THIS DEMO
#####################################################
# Define the working directory for files
my $dir = 'c:/Documents and Settings/myname/Desktop/';
# Define the filename to put OP's input
my $infilename = 'TestCase.txt';
# Define the filename to write the results into
my $outfilename = 'TestCaseOut.txt';
# Create the full-path filenames for input and output
my $infile = $dir . $infilename;
my $outfile = $dir . $outfilename;
open(INFILE,"<",$infile) ||
   die "Can't open input file $infilename: $!\n";
open(OUTFILE,">",$outfile) ||
   die "Can't open output file $outfilename: $!\n";
####################################################



# PROCESS THE INPUT

my $line = "";  # Initialize variable to hold each line
                # read in from the input file
my $prior_line = ""; # Initialize variable to hold the
                     # previous line (will need for the 
                     # swap if need to do the swap



# Process each line as it is read in, always keeping track
# of previous line (in case need to do a swap

foreach $line (<INFILE>){
   chomp($line);  # remove the trailing '\n' new-line

   # Per OP's strategy, check to see if the prior line was
   # a 'fixed-address' entry...using a regex
   if($prior_line =~ /^fixed-address/){

     # if the prior line was, indeed a 'fixed-address' then
     # check to see if this line is a 'hardware ethernet',
     # if it is then do the OP's swap using Perl's
     # ability to do the swap in-place

     ($line,$prior_line) = ($prior_line,$line)
        if($line =~ /^hardware ethernet/);
   }

   # I write the output to the output file on each
   # iteration, but since I am effectively doing a sort of
   # look-ahead one line at a time, I only write the line
   # that I'm finished with...i.e., the $prior_line

   print OUTFILE "$prior_line","\n";

   # Now, remember the line I just read in so that it 
   # becomes the $prior_line

   $prior_line = $line;
}

# When I've read in and processed the entire file, I
# have written out all of the lines except the last one
# I read in...which the above loop just moved into
# $prior_line.  So I need to write it out to the output
# file to complete the effort.

print OUTFILE "$prior_line","\n";
   
exit(0);

##</code><code>##

# Host 1
host 45583 { 
filename "junk1.cm";
hardware ethernet 11:42:a3:d4:55:83;
fixed-address 10.100.34.114;
}
 # Host 2
host D78C3 {
filename "junk5.cm";
hardware ethernet 11:42:a3:FD:78:C3;
fixed-address 10.100.34.117;
}
# Host 3
host 3A684 {
filename "junk6.cm";
hardware ethernet 11:42:a3:13:a6:84;
fixed-address 10.100.34.119;
}
# Host 4
host 46d54 {
filename "junk4.cm";
hardware ethernet 23:10:3d:14:6d:54;
fixed-address 10.100.34.120;
}