Here are my favorites:

#6 - I like the simplicity. I think it would be a good logo for the front of the shirt, but smaller than shown in the picture.
#10 - Very nicely done and would be a good choice to put on the back since it demonstrates both perl and what the Perl Monks are about.
#11 - Stresses the URL nicely and those that can read Perl, or even those who can't, can get a kick out of it.

Well that's what I think of it, so far at least.

-- zdog (Zenon Zabinski)
   Go Bells!! '¿'