On the perl-qa list tonight, we were discussing how best to find all the modules used in a source tree. To do the job right, you'd have to run the code and then look at the %INC:: hash, which shows the paths of all modules loaded. The low-tech and usually-good-enough solutions we came up with use ack:
$ ack -h '^uses+(w+(?:::w+)*).*' --output=$1 | sort -u
Thanks to Andy Armstrong for coming up with a better regex than mine, which assumed that the use statement would necessarily end with a semicolon.