You often need to do something to a line depending on what is on the next line. One way to go is to store a line, move to the next line, decide what to do and then store the current line as appropriate. 
#! /usr/perl/bin

use warnings;
use strict;

my $skip = 0;

my $last_line = <DATA>;
chomp($last_line);

while (my $current_line = <DATA>){
    chomp($current_line);
    if ($current_line =~ /^\d{4}-\d{2}-\d{2}/){
        if (not $skip){
            print $last_line, qq{\n};
        }
        else{
            $skip = 0;
        }
        
        $last_line = $current_line;
    }
    else{
        print $last_line, qq{ }, $current_line, qq{\n};
        $skip = 1;
    }
}

print $last_line, qq{\n};

__DATA__
2013-01-01 user1 user2 chat chat
2013-01-02 user3 user4 more chat more chat
2013-01-03 user5 user6 nazi nazi
chat chat nazi
2013-01-04 user 7 user8 see you in the pub

[download]
2013-01-01 user1 user2 chat chat
2013-01-02 user3 user4 more chat more chat
2013-01-03 user5 user6 nazi nazi chat chat nazi
2013-01-04 user 7 user8 see you in the pub

[download]
Boy, am I rusty. It hurt my head getting that to compile and run. :-)

Updated: Reset the $skip flag.

In reply to Re: Parse chat & File::find by wfsp
in thread Parse chat & File::find by 4perl

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.