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.