use Time::Local qw( timelocal_nocheck );

my @time_data;

@time_data = localtime();

$time_data[4]++;   # Next month.
$time_data[3] = 0; # Last day of previous month.

@time_data = localtime(timelocal_nocheck(@time_data));

my $year     = $time_data[5]+1900;
my $month    = $time_data[4]+1;    # Make 1-based
my $last_day = $time_data[3];

printf("The last day of this month (%04d/%02d) is %d.\n",
    $year, $month, $last_day
);

__END__
output
======
The last day of this month (2005/01) is 31.