$Library_Index{<$Library>} = tell($library), scalar <$Library> until eof($Library);

##</code><code>##

#! perl -slw
use strict;
use Storable qw[ store ];

print time;

my %idx;
$idx{ <> } = tell( STDIN ), scalar <> until eof STDIN;

store \%idx, '1031021.idx' or die $!;

print time;

##</code><code>##

C:\test>1031021-i.pl <1031021.dat
1367160156
1367160362

C:\test>dir 1031021*
28/04/2013  15:30               193 1031021-i.pl
28/04/2013  15:04     5,272,940,608 1031021.dat
28/04/2013  15:46           316,385 1031021.idx
28/04/2013  15:29               374 1031021.pl

##</code><code>##

#! perl -slw
use strict;
use Storable qw[ retrieve ];

print time;
my $idx = retrieve '1031021.idx' or die $!;
print time;

open DAT, '+<', '1031021.dat' or die $!;
for( 1 .. 1000 ) {
    my( $id, $offset ) = each %$idx;
    seek DAT, $offset, 0;
    my $vid = <DAT>;
    die 'mismatch' unless $id eq $vid;
    my $data = <DAT>;
}
close DAT;

print time;

##</code><code>##

C:\test>1031021.pl
1367160624
1367160624
1367160651