Usually, Internal Server Errors are because of one of these reasons:

• The script does not have the correct permissions. wakaba.pl and captcha.pl should have their permissions set to 755.
• The path to the perl executable at the top of wakaba.pl and captcha.pl is wrong. Usually, the first line should be #!/usr/bin/perl, but some servers put perl somewhere else, and then you have to change this line. You'd have to ask your host about that, though.
• A .htaccess file that uses options that the server doesn't support or allow will also make Internal Server Errors. If the host told you to do that, though, this shouldn't be a problem.

If you can see the error_log file on your server, it should tell you more about what was going wrong.