The documentation Date::Manip::Changes5to6 is very clear about this:

The words "today", "tomorrow", and "yesterday" in 5.xx referred to the time now, 24 hours in the future, and 24 hours in the past respectively.

As of 6.00, these are treated strictly as date strings, so they are the current day, the day before, or the day after at the time 00:00:00.

The string "now" still refers to the current date and time.