A) Are you using mod_perl or not?

Yes. I've been reading the CGI::Simple docs carefully, and you probably know what it says about working alongside mod_perl:

This module uses Selfloader and the __DATA__ token to ensure that only code that is used gets complied. This optimises performance but means that it will not work under mod_perl in its default configuration. To configure it to run under mod perl you would need to remove two lines from the module.

    use Selfloader;
    ....
    __DATA__
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Those two lines are commented in the source code of the module I'm using, so I guess that's enough. As an added bonus, here are the Apache config file and mod_perl startup file I'm using.

<Location /perl-status>
    SetHandler perl-script
    PerlHandler Apache::Status
    Order deny,allow
    Deny from all
    Allow from 127.0.0.1
</Location>

Alias /perl/ "/path/to/some/folder/"
<Location /perl/>
    Options ExecCGI
    SetHandler perl-script
    PerlHandler Apache::Registry
    PerlHeaderParserHandler "sub { tie *STDOUT, 'Apache' unless tied *
+STDOUT; }"
    PerlSendHeader On
    Order deny,allow
    Deny from all
    Allow from 127.0.0.1
</Location>

PerlRequire mod_perl_startup.pl
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#!/usr/bin/perl

use strict;
use lib qw(/usr/local/lib/perl); 
use CGI::Simple qw(-newstyle_urls);
use Apache::DBI();
use DBI();
use DBD::mysql();
1;
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B) The error message from CGI::Simple seems pretty explicit to me. The content-length was 31 but it couldn't read any data from STDIN. This is probably because you can only read data *once* from STDIN, so if anything has read the data from STDIN first, CGI::Simple won't then also be able to read the data.

OK, makes sense

GET type requests don't work like this, as reading the appropate %ENV variable doesn't reset anything. So check to make sure nothing is reading the STDIN before you do -- you might consider scripts started by mod_perl related directives?

As far as I can tell, I have none, except that mod_perl_startup.pl I pasted up there.

C) If you are using mod_perl, I think you want Apache::Request or something similar.

OK, will take a look at that and give it a try. Thanks for the help.