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