function get_yesterday {
  perl << "EOF"
    my ($mday, $mon, $year) = (localtime(time()-86400))[3,4,5];
    $mon++;
    $year += 1900;
    printf("%04d%02d%02d\n", $year, $mon, $mday);
EOF
}

yesterday=$(get_yesterday)
print "Yesterday was $yesterday, isn't it?"