Perl 警告："Found = in conditional, should be =="，但行上没有等号

我在Mac OS X v10.7.2(Lion(上的Perl v5.12.3中运行以下程序：

#!/usr/local/bin/perl
use strict;
use warnings;
use DBI;
my $db = DBI->connect("dbi:SQLite:testdrive.db") or die "Cannot connect: $DBI::errstr";
my @times = ("13:00","14:30","16:00","17:30","19:00","20:30","22:00");
my $counter = 1;
for (my $d = 1; $d < 12; $d++) {
    for (my $t = 0; $t < 7; $t++) {
        # Weekend days have seven slots, weekdays
        # have only four (barring second friday)
        if (($d+4) % 7 < 2 || ($t > 3)) {
            $db->do("INSERT INTO tbl_timeslot VALUES ($counter, '$times[$t]', $d);");
            $counter++;
        # Add 4:00 slot for second Friday
        } elsif (($d = 9) && ($t = 3)) {
            $db->do("INSERT INTO tbl_timeslot VALUES ($counter, '$times[$t]', $d);");
            $counter++;
        }
    }
}
$db->disconnect;

我得到了一个"；Found=在条件中，应为==在addtimes.pl行16"；警告，但这条线上没有等号。此外，循环似乎从$d == 9开始。我错过了什么？

第16行：

if (($d+4) % 7 < 2 || ($t > 3)) {