use Date::Manip; 
# this is the current month
my $current = ParseDate("today"); # or = ParseDate("january 2010") or whatever
# now let's find the last day of the previous month
$last_month_last_day = DateCalc(UnixDate($current, "%Y-%m"), "-1 day"); 
# then we print the interval
print UnixDate($last_month_last_day, "01 %b %Y - %d %b %Y\n");