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.

##</code><code>##

use Time::Local qw( timegm_nocheck );

my $year = 2005;

for my $month (1..12) {
   my $last_day = (gmtime(timegm_nocheck(
      0, 0, 0,
      1-1,        # Last day of previous month.
      $month-1+1, # Next month (0-based).
      $year
   )))[3];

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

__END__
output
======
The last day of 2005/01 is 31.
The last day of 2005/02 is 28.
The last day of 2005/03 is 31.
The last day of 2005/04 is 30.
The last day of 2005/05 is 31.
The last day of 2005/06 is 30.
The last day of 2005/07 is 31.
The last day of 2005/08 is 31.
The last day of 2005/09 is 30.
The last day of 2005/10 is 31.
The last day of 2005/11 is 30.
The last day of 2005/12 is 31.