Subtract a day? Date::Calc exists for this reason.

use Date::Calc qw[Add_Delta_Days Today];
my ($year, $month, $day) = Today();
print "Today is $year-$month-$day; ";

($year, $month, $day) = Add_Delta_Days( Today(), -1 );
print "Yesterday was $year-$month-$day!\n";

__END__
# the above code prints:
Today is 2006-6-30; Yesterday was 2006-6-29!
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This way, you'll take into account all the weirdness of dates (like leap-years, to pick a simple example) without very much work. Good luck!