at least one error in previous code ... while condition is wrong. 

use strict;
use warnings;

my @FILES;

open $FILES[0], '<', 'file1' or die;
open $FILES[1], '<', 'file2' or die;

my @w = map scalar <$_>, @FILES;

my $empty = "blank line\n";
my %map = (
    -1 => [ 0 ],
    1  => [ 1 ],
    0  => [ 0, 1 ],
);

while (defined $w[0] && defined $w[1]) {
    my $cmp = $w[0] cmp $w[1];

    print $cmp ? $empty : $w[0];
    @w[ @{$map{$cmp}} ] = map scalar <$_>, @FILES[ @{$map{$cmp}} ];
}

# Leftover lines in either file?
print map $empty, map <$_>, @FILES;

[download]

In reply to Re^3: Loop through 2 files in parallel by happy.barney
in thread Loop through 2 files in parallel by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.