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

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

my $line = 2990;
open IDX, "$source.offset";
...
open SRC, "$source";
seek SRC, $offset, 0;
print "Line $line is:\n", scalar <SRC>;