I see you already have a few responses, but here's mine anyway:

#!/usr/bin/perl -w
use strict;

open (IN, "<infile.txt") or die "$!\n";
open (OUT, ">outfile.txt") or die "$!\n";

# Read the entire file into an array
my @lines = <IN>;
close IN;

# An array evaluated in scalar context returns the number of elements
my $numlines = @lines;

# Iterate through the array, stopping at the second last element
# Array indexes start from 0, so we stop at $numlines -2
my $cnt;
for ($cnt=0; $cnt<=$numlines-2; $cnt++) {
    # Do stuff...
    $lines[$cnt] =~ # blah...blah...
}

# Index -1 always refers to the last element
# So process the last line separately here...
$lines[-1] =~ # blah...blah...

# Now, print the lot
print OUT @lines;
close OUT;
[download]

--Darren