Added perl environment (#311)

Perl runtime environment and example function.

environments/perl/README.md

@@ -0,0 +1,30 @@
+# Fission Perl environment
+
+This is an environment to execute perl code.
+
+It is implemented using the lightweight web application framework
+[Dancer2](https://metacpan.org/pod/Dancer2) and the async I/O webserver
+[Twiggy](https://metacpan.org/pod/Twiggy).
+
+Since Twiggy is implemented with [AnyEvent](https://metacpan.org/pod/AnyEvent),
+you can write async code using AnyEvent in your function.
+
+## Usage
+
+The function you upload is actually a perl package returning a coderef (see the
+example).
+
+You can use other packages and add more subs, but only the coderef you returned
+from the package is executed directly.
+
+Your main-function gets exactly one argument: the Dancer2 request object. You
+may want to `use Dancer2` to get functions like `send_error`.
+
+## Example
+
+```perl
+# hello.pm
+return sub {
+    return 'Hello, Perl!';
+}
+```