open SRC, "$source";
open IDX, ">$source.offset";
binmode IDX;
while(<SRC>) {
    print IDX pack 'N', tell SRC;
}

##</code><code>##

use CGI;
$source = $INC{'CGI.pm'};

##</code><code>##

my $line = 2990;
open IDX, "$source.offset";
binmode IDX;
seek IDX, 4 * ($line - 2), 0;
read IDX, my($packed), 4;
$offset = unpack 'N', $packed;

open SRC, "$source";
seek SRC, $offset, 0;
print "Line $line is:\n", scalar <SRC>;