$ENV{IM_ROOT} =  $TC_ROOT;   # set IM_ROOT
$ENV{IM_DATA} |= $TC_DATA;   # set IM_DATA if not already set
system qw(program -switch argument);  # program sees new values of IM_ROOT and IM_DATA

{   # this value of IM_DATA is only seen while this block is executing
    local $ENV{IM_DATA} = $TC_DATA; 
    system qw(program -switch argument);
}