I like using Time::Local whenever I can because it's fast and a core module. By combining localtime and timelocal_nocheck, you can do simple date manipulation:

#/usr/bin/perl -l
use POSIX 'strftime';
use Time::Local 'timelocal_nocheck';

my ($d, $m, $y) = (localtime)[3,4,5];
my $yesterday   = timelocal_nocheck 0, 0, 0, $d - 1,   $m, $y;
my $fivehundred = timelocal_nocheck 0, 0, 0, $d + 500, $m, $y;

print "Today:       ", strftime "%Y%m%d", localtime;
print "Yesterday:   ", strftime "%Y%m%d", localtime $yesterday;
print "In 500 days: ", strftime "%Y%m%d", localtime $fivehundred;

__END__
Today:       20041001
Yesterday:   20040930
In 500 days: 20060213
[download]

I like timelocal_nocheck for simple date addition/subtraction because it will DWYM when you ask for something like "the 0th day of October" (as we do in this example) or "the 20,000th second of January." See the POD for more info & examples.

Update: added ridiculous +500 days example