Normally REs are compiled each time just before they are executed (modulo optimizations where perl has determined that it doesn't need to recompile the RE). The /o flag means only compile the RE once, no matter what. And compiling it once only has semantic effect when there are variable substitutions to be performed before compilation.

Perhaps Randal was harsh, but that's okay if the message got through: don't use /o until you understand what it means (Or as I like to shorten it to: don't use /o :-)