I'm playing with a simple chat room program

What program? What protocols does it use?

I need the server to sometimes do nothing, ie not reply to a request. If I do this I get an error from Apache

Well, don't do that then, or get apache to ignore it, or something else

If I send a blank header the browser tries to download it as it's getting a non-executable file

A "blank header" is nonsense , apache is probably fixing the headers for you so they're not blank (nonsense) anymore ... apache documents this

can this in fact be done at all?

Sure it can, if you learn about the internet you'll know the magic words

$ perl -MCGI -e " my $q = CGI->new; print $q->header(qw/ -status 204 -
+nph 1/) "
HTTP/1.0 204
Server: cmdline
Status: 204
Date: Tue, 19 Nov 2013 13:58:00 GMT
Content-Type: text/html; charset=ISO-8859-1
[download]

...and this relates to PM, how?

Pls add code or sufficient detail to justify asking this question here, rather than on an Apache forum.

And, not just BTW, you'll probably want to specify which Apache; the circumstances in which you want Apache to "sometimes do nothing", and the error you're seeing.

It might also be wise to include a rationale for your desire -- IOW, why are you trying to do what, this way.

(I am taking for granted that the OP is addressing a Perl-based CGI or FastCGI handler ... therefore, that the question is being asked in an appropriate venue.)

Can you perhaps return 204 No Content?   (See:   this WikiPedia page...)   Now, you are fulfilling the handshake obligation of the protocol, but saying that there is nothing else to say.

You might also be interested to know that Apache(2*) already does/provides this internally. Since I wrote, and use my own HTTPD, I don't use Apache as a rule. But I do keep a version on one of my servers for experimentation. Here's an example from the log:

apache.null - - [19/Nov/2013:07:13:45 -0800] "OPTIONS * HTTP/1.0" 200 
+- "-" "Apache/2.2.24 (...) (internal dummy connection)"
[download]

HTH, and Best wishes.

--Chris

#!/usr/bin/perl -Tw
use Perl::Always or die;
my $perl_version = (5.12.5);
print $perl_version;