In the episode where Julia Roberts appears as a guest there is a part where Julia to get back at Chandler in a restaurant bathroom steals his clothes and leaves him naked. So, when he is walking with the bathroom door outside the restaurant to cover himself you can clearly see he has boxers on and he is NOT naked!!!
Chandler does ask for Joey's boxers, but Joey said he wasn't wearing any! So he shouldn't be wearing boxers, unless he's got Ross's, which wasn't mentioned in the episode. And his underwear from Julia Roberts' character is most likely a g-string according to comments made, and they definitely don't look like boxers!
I highly doubt that Matthew Perry actually wore a G-string even though in the episode Chandler is. He probably had boxer shorts on the whole time and he wouldnt want to be naked on camera. Even though Chandler is supposed to be wearing the Underwear, Matthew Perry is not really.
yeah, matthew perry wouldnt really be naked or wearing julia roberts g-string. he had to wear something. the only slip up i see here is the fact that he didnt hold the door good enough to cover his boxers.