Recently I've started to work in an evironment where there are tons of CGI script without any mention of use strict; or use warnings. At this point I don't yet have the political power to set up a PSGI based server, but as I hope one day I'll be I wanted to write my new scripts using PSGI. The problem was, how could I run my PSGI script as CGI?

Luckily Tatsuhiko Miyagawa, the author of Plack and PSGI has thought about similar needs and included the Plack::Handler::CGI module in th main Plack distribution.

Hello World!

examples/plack-cgi-hello-world.pl



#!/usr/bin/env plackup use strict; use warnings; sub { return [ '200', [ 'Content-Type' => 'text/html' ], [ 'Hello World!' ], ]; };

You probably already know from the Getting started with PSGI article that in the PSGI standard we are expected to create a subroutine and have it as the last expression in our main Perl file. That subroutine will be called on every request from the web client and it is expected to return an array reference with 3 values.

The first is the HTTP Status code which is 200 in case of success. The second is a list of key-value pairs that will become the response header. The third is the actual content of the page.

So that's what we have in our example. So far this was exactly the same as with any other PSGI code.

The difference is in the Hash-bang line. It points to plackup:

#!/usr/bin/env plackup

We can run this by typing in

perl plack-cgi-hello-world.pl

and it will launch a small web server on port 2000.

Alternatively, if we would like to run it as a CGI script, we can convert it to be executable chmod +x plack-cgi-hello-world.pl on Unix/Linux systems, and configure our web server (most likely Apache) to serve this script as a CGI script.

That's it. Now you have the skeleton of a PSGI based application, but running as a plain CGI script.

Using with Plack on other port

When I tried the above using perl plack-cgi-hello-world.pl I got an error:

failed to listen to port 5000: Address already in use at .../HTTP/Server/PSGI.pm line 94.

This means I already have another application, probably another PSGI-based application running on that port.

The way to change this sctipt to run on a different port is by adding --port to the hashbang line.

examples/plack-cgi-hello-world-port-2000.pl



#!/usr/bin/env plackup --port 2000 use strict; use warnings; sub { return [ '200', [ 'Content-Type' => 'text/html' ], [ 'Hello World!' ], ]; };

That is not enough though. If we run this as perl plack-cgi-hello-world-port-2000.pl it will still try to open port 5000. We also need to add the eXecutable bit: chmod +x plack-cgi-hello-world-port-2000.pl and run it as

./plack-cgi-hello-world-port-2000.pl