use Time::Local;

### this will get me the Unix time
### for the last day in May

# get today's month and year
my ($m, $y) = (localtime)[4,5];

# get next month's month and year
my ($nm, $ny) = ($m == 11) ? (0, $y+1) : ($m+1, $y);

# get noon on the first of next month
my $next_month_noon = timelocal(0,0,12, 1,$nm,$ny);

# subtract a day from it (86400 seconds)
my $month_last_day = $next_month_noon - 86400;