I think you'll slap your forehead for this one. (:

Does any monk have an idea why [...] the initial pipe needs to be escaped as well

For the same reason that you realized that you needed to do some escaping in the first place. /|/ matches "either the empty string or the empty string" while /\|/ and /[|]/ match a single vertical bar character.

So s#|#\\|#g replaces every empty substring of a string with '\|'.

                - tye