Sometimes you need to stop being so serious. Play. Though playing we learn new skills, and new techniques. Push boundaries - see what is possible. Most importantly we can demonstrate new things in a memorable way.
Source filters are a important feature in Perl. For the first time it is possible to dynamically alter your code before it's interpreted, and by this alter the meaning your code has. No other feature has before offers the power to rebuild the language in any way imaginable.
So what do we do with this power? Why, we play of course. Rather than having source code that looks like - well, Perl - we have source code that looks like a picture of a Pony. And why not? What an exciting way to learn about new tools. Of course, it's all Damian's fault for showing London.pm Bleach.pm.
As an aside, no I don't know where the whole "I want a pony" meme stated in the Perl community. Sometimes it's best not to ask.