You wouldn't write $index = $index + 1 in bash.

Here's some untested code in Perl:

use autodie;
use Fcntl qw[:DEFAULT :seek];
sysopen my $fh, "/tmp/text.index", O_RDWR | O_CREATE, 0666;
my $count = <$fh> || 0;
seek $fh, 0, SEEK_SET;
truncate $fh;
print $fh $count + 1;
close $fh;
[download]

Or use a precanned CPAN solution.