fix plan

the plan can not be computed at import time, see following warnings:
    t/INSTALL_BASE............Use of uninitialized value in addition (+) at t/INSTALL_BASE.t line 21.
    Use of uninitialized value in addition (+) at t/INSTALL_BASE.t line 21.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>