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
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
Boy, am I rusty. It hurt my head getting that to compile and run. :-)
Updated: Reset the $skip flag.