Hello, adapting your code: 

#!/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

[download]
Output: 
foo: 1 now changed to 2
bar: 2 now changed to 4
baz: 3 now changed to 7
qux: 4 now changed to 11

[download]

You may want to look at Text::CSV_XS for this kind of task.

Hope this helps!

The way forward always starts with a minimal test.

In reply to Re: How to loop over two lines, alter a value in the current line and save it to the previous line? by 1nickt
in thread How to loop over two lines, alter a value in the current line and save it to the previous line? by rjc33

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