We made a Powered by Perl button with the new Perl logo some time ago.
This is the only version of the onion that you are allowed to use and it must link to www.perl.org.
<a href="http://www.perl.org/"> <img src="http://i.perl.org/powered/perlpowered.png" border="0" width="134" height="48"></a>
The Onion is a trademark of the Perl Foundation (like O'Reilly owns the camel). In the not too far future there will be usage and trademark information page for the onion logo at perl.org or the TPF site. But, at least until then - no using the onion logo for your Perl Monger group. Or anything else. Only use the "powered by Perl" button, please. :-)
Copy the .png to your own server, or use image on the i.perl.org server.