Date::Calc's Day_of_Week function will do exactly what you want. I have the following snippet which I used once before when I could not install Date::Calc. Unfortunately I did not record it's provenance and I googled around and couldn't find it :( I seem to recall it coming from one of our esteemed monks though...

# Return the day (1..7) that the first day of the given month/year fal
+ls
# on. Uses "Zeller's Confluence", which I don't claim to understand.
#
sub Day_of_Week {
  my($year, $month, $day) = @_;   # $month in (1..12), $year as YYYY
  $month-=1;
  if ( $month < 2 ) {
    $month += 12;
    --$year;
  }
  my $z1 = (26 * ($month + 2)) / 10;
  my $z2 = int((125 * $year) / 100);
  my $day_of_week = ($z1 + $z2 - int($year / 100) + int($year / 400)) 
+% 7;        
  return $day_of_week ? $day_of_week : 7;
}
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It would need a wrapper to do what you want, subtract one (previous day) and wrap to seven if result zero...