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

my $previous_line = <DATA>;

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

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

    print "$current_cols[0]: $current_cols[1] ";

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

    print "now changed to $current_cols[1]\n";

    $previous_line = join ' ', @current_cols;
}

__DATA__
seed 1
foo 1
bar 2
baz 3
qux 4

##</code><code>##

foo: 1 now changed to 2
bar: 2 now changed to 4
baz: 3 now changed to 7
qux: 4 now changed to 11