This technique works for me as well. I'm actually using it in a mod_perl app server to reload user modules upon change. I snagged the code right out of Apache::StatINC. If you look at Apache::Symbol, it makes special provisions for undefining exported functions and this may be why you aren't seeing the changes.

/\/\averick
perl -l -e "eval pack('h*','072796e6470272f2c5f2c5166756279636b672');"