Sorry if these aren't the best ideas but most of the good ones are taken already...
How about:

• 24-CD holder case thingy (for the Perl CD-ROMS, etc...)
• Perlmonk's shirt in 0x000000 (black)
• bookmark or those little post-it booktags (to find that section in camel3 faster)
• thermal mug (coffee or cold caffeinated quaff  ;-)

Also, the (self-bought) gift for making different levels could be enchanting, but it would make sense to start allowing those at level 4 or somewheres about there.

nuf evah,
jynx

PS   i really like the boxers idea, but in my idiom, could they be delivered in black as well?