Be sure that you put this construction in double quotes not single ones. (Double-quotes cause interpolation; single-quotes suppress it.) And be sure also that you are sending the value (in an insert or update query) using a query-parameter.
thanks for replies, i have number of fields which has similar values. so i wanted it to work this way. However Corion's tips was usefull and workable. while doing experiment i found a way. In mySQL table i have modified the value from $ENV{INP_DIR} to "$ENV{INP_DIR}" double quotes included in value itself and it worked.