#! perl -slw
use strict;

my %idx; ## Index the file
$idx{ <> } = tell( ARGV ), scalar <> until eof();

for ( 1 .. 1000 ) {
    my $id = getNextId( ... );
    seek ARGV, $idx{ $id };
    scalar <>; # discard id line (or verify)
    print scalar <>; ## access data;
}