#!/usr/bin/perl
use strict;
use warnings;

my $previous_line = <DATA>;

print "        prev\tcurrent\n";

while ( my $current_line = <DATA> ) {
    chomp for ( $previous_line, $current_line );

    print "input:\t$previous_line\t$current_line\n";

    my @previous_cols = split ' ', $previous_line;
    my @current_cols  = split ' ', $current_line;

    $current_cols[1] += $previous_cols[1];

    $previous_line = $current_line = join ' ', @current_cols;

    print "output:\t$previous_line\t$current_line\n";
    print "\n";
}

__DATA__
nul 0
foo 1
bar 2
baz 3
qux 4