The Open Source Convention (OSCON) started out as the Perl Conference (TPC). It is put on by O'Reilly and it is still the prime Perl conference even though it shares the stage with other open source projects. I have attended all but one of these conferences and given several talks at them. It is professionally run and it does cost a fair amount so mostly those whose companies can pay for them attend it.
Yet Another Perl Conference (YAPC) was started in 1999 as a cheaper alternative to TPC/OSCON. As such it attracts many students and self employed Perl hackers who can't afford the other conference. It has grown since then and there are YAPC's all over the world now. I have been to all the YAPC::NA conferences and in 2003, I went to YAPC::Paris (of course the wife tagged along!). I have given talks at most of the YAPC's I have attended and I help out in various ways. I am now the 'official' social director of YAPC::NA which means I have the power to affect your fun, so be nice to me! Read a diary of my 1999 YAPC::NA experience if you dare!
Possibly the most fun I have ever had at a Perl conference was on the two Perl Whirls I attended (with my wife, of course!). The first was to Alaska and the second was to the Caribbean. One time in a talk someone spotted a whale swimming and leaping in the distance. We all ran to the window and watched and the speaker joined us too. That is not going to happen in some hotel! Here is a Wired article about the first Perl Whirl cruise.