#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
use File::ReadBackwards;

my $numlines = 20;
my $filename = 'in.log';

my $bw = File::ReadBackwards->new($filename)
    or die "can't read $filename $!";

my @lines;
my $count = 0;
while( defined( my $line = $bw->readline ) ) {
    chomp $line;
    push @lines, $line;
    $count++;
    last if $count == $numlines;
}

@lines = reverse @lines;

print Dumper \@lines;

__END__

$ perl reverse_file.pl
$VAR1 = [
          'line 2',
          'line 3',
          'line 4',
          'line 5',
          'line 6',
          'line 7',
          'line 8',
          'line 9',
          'line 10',
          'line 11',
          'line 12',
          'line 13',
          'line 14',
          'line 15',
          'line 16',
          'line 17',
          'line 18',
          'line 19',
          'line 20',
          'line 21'
        ];

##</code><code>##

line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9
line 10
line 11
line 12
line 13
line 14
line 15
line 16
line 17
line 18
line 19
line 20
line 21

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
use File::ReadBackwards;

# import LOCK_* and SEEK_END constants
use Fcntl qw(:flock SEEK_END);

sub lock {
    my ($fh) = @_;
    flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
    # and, in case someone appended while we were waiting...
    seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
}

sub unlock {
    my ($fh) = @_;
    flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
}

my $numlines = 20;
my $filename = 'in.log';

my $bw = File::ReadBackwards->new($filename)
    or die "can't read $filename $!";

my $fh = $bw->get_handle();

lock($fh);

my @lines;
my $count = 0;

while( defined( my $line = $bw->readline ) ) {
    chomp $line;
    push @lines, $line;
    $count++;
    last if $count == $numlines;
}

unlock($fh);

$bw->close();

@lines = reverse @lines;

print Dumper \@lines;

__END__

$ perl reverse_file.pl
$VAR1 = [
          'line 2',
          'line 3',
          'line 4',
          'line 5',
          'line 6',
          'line 7',
          'line 8',
          'line 9',
          'line 10',
          'line 11',
          'line 12',
          'line 13',
          'line 14',
          'line 15',
          'line 16',
          'line 17',
          'line 18',
          'line 19',
          'line 20',
          'line 21'
        ];

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
use File::ReadBackwards;

# import LOCK_* and SEEK_END constants
use Fcntl qw(:flock SEEK_END);

sub lock {
    my ($fh) = @_;
    flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
    # and, in case someone appended while we were waiting...
    seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
}

sub unlock {
    my ($fh) = @_;
    flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
}

sub check_file {
    my ($file, $numlines) = @_;
    my $bw = File::ReadBackwards->new($file)
	or die "can't read $file $!";

    my $fh = $bw->get_handle();

    lock($fh);

    my @lines;
    my $count = 0;

    while( defined( my $line = $bw->readline ) ) {
	chomp $line;
	push @lines, $line;
	$count++;
	last if $count == $numlines;
    }

    unlock($fh);

    $bw->close();

    return reverse @lines;
}


my @data;
my $numlines = 20;
my $process = "line 2";
my $filename = 'in.log';

while ( 1 ) {
    @data = check_file($filename, $numlines);
    last if (grep /$process/, @data);
    sleep 5;
}

print Dumper \@data;

__END__

$ perl reverse_file.pl
$VAR1 = [
          'line 2',
          'line 3',
          'line 4',
          'line 5',
          'line 6',
          'line 7',
          'line 8',
          'line 9',
          'line 10',
          'line 11',
          'line 12',
          'line 13',
          'line 14',
          'line 15',
          'line 16',
          'line 17',
          'line 18',
          'line 19',
          'line 20',
          'line 21'
        ];

##</code><code>##

do {
    @data = check_file($filename, $numlines);
    sleep 5;
} while ( grep not /$process/, @data );