Well, first off your code reads from STDIN like a c program. CGI is easier to use. Here's your code:

read(STDIN, $buffer, $ENV{'CONTENT_LENGTH'});

$rating = substr($buffer,0,1);
$table = substr($buffer,1,(length($buffer)+1));
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Here's mine that does the same thing:

use CGI qw/:standard/;
my ($rating, $table);
if (param()){
 $rating = param('rating');
 $table = param('table');
}
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Try reading the CGI documentation by using perldoc CGI

Especially read the section on debugging on the command line:

perl -e "use CGI qw/:standard :debug/; if(param()){print  param('you')
+};" you=hello
hello
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And before you are done you should read about tainted data. Your program takes information from the user and turns around and tries to write to a file. That is dangerous:

perl -Te "use CGI qw/:standard :debug/; $ms=param('you'); if (-f qq($m
+s\.txt)){open(H,qq(>>$ms\.txt))};print <H>; close(H);" you=hello
Insecure dependency in open while running with -T switch at -e line 1.
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