If you are on unix you can use "cal"

my ($mon,$year) = (localtime())[4,5];
$year +=1900;

# perl counts months begining at 0 while cal begins at 1 so 
# the month returned  from perl's localtime is already last 
# month, we just need to fix for jan
$mon = 12 if ! $mon;

my $last_day_last_month=(split /\s/, `/bin/cal $mon $year`)[-1];
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I don't believe this solution has any time zone problems either... :)

--
Life is a tale told by an idiot -- full of sound and fury, signifying nothing. William Shakespeare, Macbeth