my $yesterday = time() - ( 24 * 60 * 60 ); ## Why use 'my' here #################

## And not here -v-v-v-v-v-v-v-v- #####################
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($yesterday);
$year += 1900;
$mon++;
$d = "$mday"; ## <<< Why copy $mday to $d? And why quote "$mday"?  #########################
if ($d <10) {
        $d = "0$d"; ## <<< What does this do? #################
}
$m ="$mon";
if ($m<10) {
        $m = "0$m"; ## <<< And this? ####################
}
$yesterday = "$year" . "$m" . "$d"; ## Why all those "s? #######