If you're after a "best" implementation ... tie::file

There's a couple of problems with that.

1. Performance:

   Writing a file with Tie::File, even with memory allocated to easily accommodate the whole file, is orders of magnitude slower than direct writing.

   c:\test>junk7 -N=10e3 ### 1/2 MB
   Took 0.098 seconds
   Took 34.255 seconds
   
   c:\test>junk7 -N=20e3 ### 1 MB
   Took 0.197 seconds
   Took 137.506 seconds
   
   c:\test>junk7 -N=1e6 ### 50 MB
   Took 9.449 seconds
   ^C
   [download]

   By the time you get to 50 MB I estimate it will take hours instead of 10 seconds.

2. There doesn't seem to be any simple way to binmode a Tie::File tied file. Which means that on some systems, the data in the file will be different to that checksummed:

   21/07/2010  01:26           510,033 junk.dat
   21/07/2010  01:26           520,034 junk2.dat
   [download]

Test code:

#! perl -slw
use strict;
use Time::HiRes qw[ time ];
use Tie::File;
use Digest::MD5 qw[ md5_hex ];

our $N //= 1e6;

my $start = time;
open OUT, '+>:raw', 'junk.dat';
print OUT md5_hex( 0 );

my $data = 'x'  x 50;
my $md5 = new Digest::MD5;
for ( 1.. $N ) {
    print OUT $data;
    $md5->add( "$data\n" );
}

seek OUT, 0, 0;
print OUT $md5->hexdigest;
close OUT;
printf "Took %.3f seconds\n", time-$start;

$start = time;
tie my @lines, 'Tie::File', 'junk2.dat', memory => 52 * $N;

$md5 = new Digest::MD5;
push @lines, md5_hex( 0 );
for ( 1.. $N ) {
    push @lines, $data;
    $md5->add( "$data\n" );
}

$lines[ 0 ] = $md5->hexdigest;
untie @lines;
printf "Took %.3f seconds\n", time-$start;
[download]