use File::Marker;

my $file = File::Marker->new( $filename );

# make the (zero-based) index 
my $i = 0;
while ( ! $file->eof ) {
    $file->set_marker($i++);
    <$file>;
}

# save and rewind
$file->save_markers( $index_filename );
$file->goto_marker('LAST'); # rewind