Check out Time::Local. This does the opposite of localtime, so you can get the day of week, etc.

use Time::Local;
my $time = timelocal($sec,$min,$hour,$mday,$mon,$year);
my @time_array = localtime($time);
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As for

date("t", mktime(0,0,0,$month,1,$year));
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I think you may have to just make an array with the last days of every month and use $lastdays[$mon] to get that day. I'm not aware of a perl function that will do that.