I've found Date::Manip to be particularly useful for complex date functions.

if ((localtime(time))[6] == 0) {       # Sunday
    print Date::Manip::ParseDate('next Monday');
    print Date::Manip::DateCalc('next Monday','+ 4 business days');
} elsif ((localtime(time))[6] == 1) {  # Monday
    print Date::Manip::ParseDate('today');
    print Date::Manip::DateCalc('today','+ 4 business days');
} else {
    print Date::Manip::ParseDate('last Monday');
    print Date::Manip::DateCalc('last Monday','+ 4 business days');
}
[download]

UPDATE: Added consideration for running the code on Monday. I don't think there are any potential implications of a MTWTFSS vs a SMTWTFS week (Saturday is still closer to "last" week; Sunday is still closer to "next" week).