c:\test>perl -e"printf qq[Line %010d\n], $_ for 1 .. 25" > junk.dat

c:\test>type junk.dat
Line 0000000001
Line 0000000002
Line 0000000003
Line 0000000004
Line 0000000005
Line 0000000006
Line 0000000007
Line 0000000008
Line 0000000009
Line 0000000010
Line 0000000011
Line 0000000012
Line 0000000013
Line 0000000014
Line 0000000015
Line 0000000016
Line 0000000017
Line 0000000018
Line 0000000019
Line 0000000020
Line 0000000021
Line 0000000022
Line 0000000023
Line 0000000024
Line 0000000025

##</code><code>##

c:\test>type indexFile.pl
#! perl -sw
use strict;

open INDEX, '>:raw', "$ARGV[ 0 ].idx" or die $!;
syswrite INDEX, pack( 'N', 0 ), 4;
syswrite INDEX, pack( 'N', tell *ARGV ), 4 while <>;
close INDEX;


c:\test>indexFile junk.dat

c:\test>dir junk.dat*
21/12/2011  17:45               425 junk.dat
21/12/2011  17:46               104 junk.dat.idx
c:\test>

##</code><code>##

c:\test>type readIndexedFile.pl
#! perl -sw
use strict;
use Time::HiRes qw[ time ];

our $N //= 100;

open INDEX, '<:raw', "$ARGV[ 0 ].idx" or die $!;
my $len = -s( INDEX );
sysread INDEX, my( $idx ), $len;
close INDEX;

sub getRecordN {
    my( $fh, $n ) = @_;
    seek $fh, unpack( 'N', substr $idx, ($n-1) * 4, 4 ), 0;
    return scalar <$fh>;
}

open DAT, '<', $ARGV[ 0 ] or die $!;

for my $line ( 1 .. ( length( $idx ) / 4 ) - 1 ) {
    print "Expecting $line; got: ",  getRecordN( *DAT, $line );
}

c:\test>readIndexedFile junk.dat
Expecting 1; got: Line 0000000001
Expecting 2; got: Line 0000000002
Expecting 3; got: Line 0000000003
Expecting 4; got: Line 0000000004
Expecting 5; got: Line 0000000005
Expecting 6; got: Line 0000000006
Expecting 7; got: Line 0000000007
Expecting 8; got: Line 0000000008
Expecting 9; got: Line 0000000009
Expecting 10; got: Line 0000000010
Expecting 11; got: Line 0000000011
Expecting 12; got: Line 0000000012
Expecting 13; got: Line 0000000013
Expecting 14; got: Line 0000000014
Expecting 15; got: Line 0000000015
Expecting 16; got: Line 0000000016
Expecting 17; got: Line 0000000017
Expecting 18; got: Line 0000000018
Expecting 19; got: Line 0000000019
Expecting 20; got: Line 0000000020
Expecting 21; got: Line 0000000021
Expecting 22; got: Line 0000000022
Expecting 23; got: Line 0000000023
Expecting 24; got: Line 0000000024
Expecting 25; got: Line 0000000025