And if you want to stick to modules that are included in the Perl core (so no trip to CPAN), you can accomplish this with Time::Local's timelocal and POSIX's fab-oo strftime ...

use Time::Local;
use POSIX qw/ strftime /;

my $today     = '10/24/2000';

my $days_ago  = 1;

my $yesterday = do {

    my($m,$d,$y)  = split( m'/' => $today );

      # I use 12 as the hour here, so there's no 
      # confusion about daylight savings.

    my $t  = timelocal( 0, 0, 12, $d, $m - 1, $y - 1900 );
       $t -= $days_ago * 24*60*60 ;

    strftime( '%m/%d/%Y' => localtime($t) );
};

print "$today $yesterday";
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Looks a lot like the Time::Piece (alt.) example, no? :)

    --k.