mktime is a POSIX function, so you can use POSIX::mktime. Although you can (probably should) use one of the Date modules from CPAN, here are a couple of brief functions:

sub dayofweek {
  # 0=Sunday .. 6=Saturday
  # input is Unix epoch time
  return (4 + int($_[0]/86400)) % 7;
}

sub daysinmonth {
  # input is Unix epoch time
  my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
      localtime($_[0]);
  if (++$mon == 12) {
    $mon = 0;
    $year++;
  }
  ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
      localtime(POSIX::mktime(0,0,0,1,$mon,$year) - 3600*3);
  return $mday;
}
[download]

Update:Errr, don't use the day of week function. It gives the day of week all right--in London (gmtime)! Here's an even simpler function that works with your time zone:

sub dayofweek { (localtime $_[0])[6] }
[download]

Duh.