Question

Possible Duplicate:
How can I read lines from the end of file in Perl?

First read the last line, and then the last but one, etc. The file is too big to fit into memory.

Answer 1

Reading lines in reverse order:

use File::ReadBackwards;

my $back = File::ReadBackwards->new(shift @ARGV) or die $!;
print while defined( $_ = $back->readline );

I misread the question initially and thought you wanted to read backward and forward, in alternating fashion — which seems more interesting. 🙂

use strict;
use warnings;
use File::ReadBackwards ;

sub read_forward_and_backward {
    # Takes a file name and a true/false value.
    # If true, the first line returned will be from end of file.
    my ($file_name, $read_from_tail) = @_;

    # Get our file handles.
    my $back = File::ReadBackwards->new($file_name) or die $!;
    open my $forw, '<', $file_name or die $!;

    # Return an iterator.
    my $line;    
    return sub {
        return if $back->tell <= tell($forw);
        $line = $read_from_tail ? $back->readline : <$forw>;
        $read_from_tail = not $read_from_tail;
        return $line;
    }

}

# Usage.    
my $iter = read_forward_and_backward(@ARGV);
print while defined( $_ = $iter->() );